Absorbent resin, absorbent material and method of manufacturing the same

ABSTRACT

A water content of a water-containing gel of a hydrophilic cross-linked polymer is reduced by pressuring (calendaring) it. Then, a gel composition including, for example, a water-containing gel including cross-linked poly(meth)acrylic acid (salt), glycerol (polyhydric alcohol) and polyester fibers (auxiliary forming material), etc., is fed to a drum dryer on an upstream side of a pressurizing roller. Then, the resulting gel composition is pressurized (calendared) heated by a pressurizing roller to be formed into a sheet. As a result, a sheet-like absorbent material having at least one smooth surface can be obtained. The absorbent material resulting from the described method shows excellent properties and especially absorbing properties such as absorbing rate, absorbency under load, and shape retaining properties.

FIELD OF THE INVENTION

The present invention relates to absorbent resins and absorbentmaterials which are suited for use in sanitary materials such as paperdiapers (disposable diapers), sanitary napkins, so-called incontinencepads, etc., moisture condensation absorbent sheets, agricultural waterretaining materials, waterproofing agents for civil engineering works,medical materials, such as medical sheets, etc., materials for keepingfoodstuffs fresh, and materials for preventing foodstuffs from dripping,etc., and a method of manufacturing such absorbent material.

BACKGROUND OF THE INVENTION

Recently, absorbent materials have been used in a variety of fields, forexample, as materials for paper diapers, sanitary napkins, so-calledincontinence pads, for the purpose of absorbing body fluids. Generally,such absorbent materials are produced in the following manner. Afterpowdery or granular absorbent resin is sandwiched between layers ofpaper, etc., processing operations such as an embossing operation areapplied to the paper, etc., or after incorporating absorbent resin withpulp, etc., to form a sheet, film, etc., processing operations such asan embossing operation are applied to the sheet, etc. Instead of thedescribed processing operation, the absorbent resin may be envelopedinto a base material using a thermoplastic resin.

A method of manufacturing an absorbent material by forming an absorbentresin into a sheet or a film is disclosed, for example, in JapaneseUnexamined Patent Publication No. 141357/1978 (Tokukaisho 53-141357) andU.S. Pat. No. 4,066,583 wherein a mixture of dried powdery absorbentresin and polyvalent alcohol is sandwiched between fluororesin sheets(base materials), and then a pressure is applied thereto. JapaneseUnexamined Patent Application No. 174414/1991 (Tokukaihei 3-174414) andU.S. Pat. No. 5,145,906 disclose absorbent materials produced from driedpowdery absorbent resin made of polyacrylic acid (salts) andpolysaccharides, etc., and paper diapers as an absorbent article.

Furthermore, Japanese Unexamined Patent Publication No. 230671/1989(Tokukaihei 1-230671) and U.S. Pat. No. 4,826,880 disclose a method offorming hydrate by adding an aqueous solution in an amount of from 20percent by weight to 80 percent by weight based on a total amount todried powdery absorbent resin and immobilizing the dried powderyabsorbent resin to a base material by extrusion and dispersion. U.S.Pat. No. 5,428,076 discloses a method of immobilizing dried powderyabsorbent resin to a base material to be formed into a sheet.

However, according to the described manufacturing method of JapaneseUnexamined Patent Publication No. 141357/1978 (Tokukaisho 53-141357),U.S. Pat. No. 4,066,583, Japanese Unexamined Patent Publication No.174414/1991 (Tokukaihei 3-174414), and U.S. Pat. No. 5,145,906, thewater-containing gel resulting from a polymerization reaction is dried,pulverized and classified to form powdery absorbent resin, and theabsorbent resin is formed into a sheet or film by incorporating theabsorbent resin with the base material. Therefore, the described methodshave a problem in that dust generated when forming the absorbent resininto powders makes the handing of the absorbent resin difficult, andworking conditions undesirable. Moreover, the yield of the absorbentresin, and the yield of the resulting molded article are lowered.Besides, complicated processes of manufacturing the absorbent materialare required, and the absorbent material cannot be manufactured at lowcost.

According to the described conventional manufacturing processes, it isrequired to reduce an amount of use of the absorbent resin particles tobe mixed in the fabric matrix in order to prevent possible gel blockingbetween absorbent resin particles. Therefore, it is difficult todesirably manufacture an absorbent material having a high content ofabsorbent resin.

Moreover, the absorbent materials resulting from the conventionalmethods are inferior in their flexibility, strength, etc. Therefore,such absorbent materials are not tolerable to be taken in a roll, etc.,or against tension, and it is not possible to manufacture the absorbentmaterials continuously. Furthermore, when manufacturing sanitarymaterials (absorbent articles) such as paper diapers, etc., from theabsorbent materials, the resulting sanitary materials are not soft andare uncomfortable to skin of the user.

According to the methods of Japanese Unexamined Patent Publication No.230671/1989 (Tokukaihei 1-230671), U.S. Pat. No. 4,826,880 and U.S. Pat.No. 5,428,076, after once forming the dried powdery absorbent resin, itis formed into a sheet. Materials such as non-woven fabrics areseparately required for maintaining the absorbent resin in a sheet form,and a complicated process of incorporating the material into theabsorbent resin powders is required. Besides, dust is generated whenforming the absorbent resin into a sheet which causes inconvenience inthe handling of the absorbent resin. Moreover, it is likely that theresulting absorbent materials are inferior in flexibility of the sheet,absorbing rate, absorbency under load, and shape retaining property ofthe sheet which makes the weight of the absorbent resin per unit areasmall, and desirable absorbing properties cannot be obtained.

The present invention is achieved in a hope of finding a solution to theabove-mentioned problems, and accordingly, a main object of the presentinvention is to provide an absorbent resin and an absorbent materialwhich offer excellent absorbing properties such as absorbing rate,absorbency under load, etc., and a shape retaining property. Anotherobject of the present invention is to provide a method of manufacturingthe absorbent material which permits the absorbent material to bemanufactured at low cost.

DISCLOSURE OF THE INVENTION

Earnest researches have been made by the inventors of the presentinvention to achieve the above-mentioned object, and they have foundthat by reducing the water content of a water-containing gel of across-linked polymer under an applied pressure, an absorbent materialcan be manufactured at low cost, and that the resulting absorbentmaterial includes an absorbent resin which is swollen by absorbing waterso as to have anisotropy, and has excellent absorbing properties such asabsorbing rate, absorbency under load, shape retaining property, etc.,to complete the present invention.

The inventors of the present invention have also found that by applyingpressure to, i.e., compressing, a cellular gel of a hydrophiliccross-linked polymer while reducing water content when necessary,absorbent materials can be manufactured at low cost, and the resultingabsorbent materials include an absorbent resin which is swollen byabsorbing water so as to have anisotropy, and have excellent absorbingproperties such as absorbing rate, absorbency under load, shaperetaining property, etc.

In order to accomplish the described objects, the absorbent resin of thepresent invention is characterized by being swollen by absorbing waterso as to have anisotropy.

Specifically, in order to accomplish the object of the presentinvention, an absorbent material of the present invention ischaracterized by including an absorbent resin, which is formed into asheet having a flexibility of not more than 1,000 mgf.

In order to accomplish the object of the present invention, a method ofmanufacturing an absorbent material of the present invention ischaracterized by reducing a water content of a hydrophilic cross-linkedpolymer under an applied pressure.

In order to accomplish the object of the present invention, anothermethod of manufacturing an absorbent material of the present inventionis characterized by applying a pressure to a cellular gel of ahydrophilic cross-linked polymer while reducing a water content whennecessary.

The present invention will describe the present invention in detail.

The absorbent material of the present invention is characterized byhaving a distorted cross-linked structure and including an absorbentresin (for example, hydrophilic cross-linked polymer particles) which isswollen (anisotropic swelling) by absorbing water. Such absorbentmaterial can be obtained, for example, by (i) a method of reducing awater content of a water-containing gel of a hydrophilic cross-linkedpolymer under an applied pressure, or (ii) a method of reducing thewater content of a cellular gel of a hydrophilic cross-linked polymerunder an applied pressure.

In the present invention, the water-containing gel of the hydrophiliccross-linked polymer suggests a gel which is swollen by absorbingaqueous solvent such as water, etc. On the other hand, the cellular gelof the hydrophilic cross-linked polymer suggests a water-containing gelof the hydrophilic cross-linked polymer, which has cells or pores(space) inside.

The cellular gel of the hydrophilic cross-linked polymer can be obtainedby introducing foams inside the water-containing gel of the hydrophiliccross-linked polymer. Cells can be introduced inside thewater-containing gel of the hydrophilic cross-linked polymer, forexample, by boiling the water-containing gel of the hydrophiliccross-linked polymer. However, it is preferable to adopt a method ofusing a foaming agent when manufacturing the water-containing gel of thehydrophilic cross-linked polymer in view of physical properties.Additionally, the method of manufacturing the water-containing gel andthe cellular gel are not particularly limited.

Such water-containing gel of the hydrophilic cross-linked polymer can beobtained with ease by carrying out a polymerization reaction of amonomer component containing, for example, ethylenically unsaturatedmonomer using an aqueous solvent as a solvent.

The cellular gel of the hydrophilic cross-linked polymer can be obtainedwith ease by carrying out a polymerization reaction in a presence of afoaming agent using an aqueous solvent as a solvent. For the aqueoussolvent, it is not limited to but is preferable to adopt water.

For the ethylenically unsaturated monomer, a water-soluble compound ispreferable.

Examples of such ethylenically unsaturated monomer include: unsaturatedcarboxylic acid such as (meth)acrylic acid, β-acryloyloxypropionic acid,maleic acid, maleic anhydride, fumaric acid, crotonic acid, itaconicacid, or the above acids in a neutralized form (salts), etc.; anionicmonomers, such as 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid,vinyl sulfonic acid, styrene sulfonic acid, and salts thereof, monomershaving a nonionic hydrophilic group such as (meth)acrylamide,N-substituted (meth)acrylamide, 2-hydroxyethyl(meth)acrylate,2-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol(meth)acrylate,polyethylene glycol mono(meth)acrylate, etc.; monomers having an aminogroup such as N,N-dimethylaminoethyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylamide, etc., and quaternary saltsthereof. Only one kind of the ethylenically unsaturated monomer may beadopted, or two or more kinds thereof may be suitably mixed and adopted.

In consideration of the absorbing properties of the absorbent materialamong the above-listed ethylenically unsaturated monomers, it ispreferable to use a compound of at least one kind selected from thegroup consisting of (meth)acrylic acids, and neutralized materialsthereof (hereinafter referred to as (meth)acrylic acid (salts)),2-(meth)acryloylethane sulfonic acid (salts),2-(meth)acrylamide-2-methylpropanesulfonic acid (salts),(meth)acrylamide, methoxypolyethyleneglycol(meth)acrylates,N,N-dimethylaminoethyl(meth)acrylates and quaternary salts ofN,N-dimethylaminoethyl(meth)acrylates. A compound of at least one kindcontaining (meth)acrylic acid (salts) is still more preferable. When theethylenically unsaturated monomer contains (meth)acrylic acid (salts),it is the most preferable that from 0 mole percent to 90 mole percent of(meth)acrylic acid is neutralized with a basic substance. Furthermore,when the neutralization ratio of (meth)acrylic acid is not more than 50mole percent, it is preferable that the resulting polymer is neutralizedwith a basic material in a water containing gel state. Namely, it ispreferable that the hydrophilic crosslinked polymer includescross-linked poly(meth)acrylic acid (salts).

Examples of the basic material include but are not limited to: sodiumhydroxide, potassium hydroxide, sodium carbonate, sodiumhydrogencarbonate, ammonia, ethanolamine, etc.

The described monomer component may contain other monomer that iscopolymerizable with the ethylenically unsaturated monomer (hereinafterreferred to as a copolymerizable monomer) to an extent that thehydrophilic characteristics of the hydrophilic crosslinked polymer donot suffer substantially. Examples of such copolymerizable monomerinclude but are not limited to: (meth)acrylates such asmethyl(meth)acrylate, ethyl(meth)acrylate, butyl(meth)acrylate, etc.;hydrophobic monomers such as vinyl acetate, vinyl propionate, etc. Onlyone kind of the above-listed copolymerizable monomer may be adopted, ortwo or more kinds thereof may be suitably mixed and adopted.

By carrying out a polymerization reaction of the monomer component usingan aqueous solvent, the gel-like hydrophilic polymer, i.e., anon-cellular gel of the present invention can be achieved. On the otherhand, in the polymerization reaction, by adopting the foaming agentduring a polymerization or after the polymerization, a water-containinggel having cells inside, i.e., the cellular gel of the present inventioncan be achieved. Hereinafter, the non-cellular gel is simply referred toas a water-containing gel if not specified, and the water-containing geland the cellular gel are generally referred to simply as gels.

Examples of the foaming agent to be used in the present invention may bebut are not limited to: organic solvent such as methyl alcohol,cyclohexane, etc.; carbonates such as sodium (hydrogen)carbonate,ammonium (hydrogen)carbonate, carbonate, potassium (hydrogen)carbonate,magnesium carbonate, carbon dioxide, ethylene carbonate, etc.;water-soluble azo compounds such as2,2'-azobis(2-methylpropionamizine)dihydrochloride,2,2'-azobis(2-(2-imidazoline-2-il)propane)dihydrochloride,2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propioneamide], etc.; and wateruniformly dispersed azo compounds such as 2,2'-azobis(2-methylpropioneamizine)diacrylate, etc. These foaming agents may be solid,liquid or gas at room temperature. From the view point of controllingfoaming, a water-soluble polymer or a surfactant may be used incombination.

Although the respective suitable amounts of use of the foaming agent,the water-soluble polymer or the surfactant vary, it is normally basedon the total amount of the monomer component, not more than 200 percentby weight, preferably not more than 100 percent by weight in the case ofcarbonates; not more than 5 percent by weight, preferably not more than1 percent by weight in the case of the azo compound; not more than 10percent by weight, preferably not more than 5 percent by weight in thecase of the water soluble polymer, and not more than 2 percent by weightand more preferably not more than 1 percent by weight in the case of thesurfactant.

When polymerizing the monomer component, the initialization may beeffected by a method of adopting a polymerization initiator or a methodof adopting an activated energy ray such as radio active ray, anelectron beam, an ultraviolet light, or an electromagnetic ray. For thepolymerization initiator, for example, radical polymerization initiatorsmay be adopted. Examples of such radical polymerization initiatorsinclude but are not limited to: inorganic peroxides such as, potassiumpersulfate, ammonium persulfate, sodium persulfate, and hydrogenperoxide; organic peroxides such as t-butyl hydroperoxide, benzoylperoxide, and cumene hydroperoxide; and azo compounds such as2,2'-azobis (N,N'-dimethylene isobutyl amidine) and salts thereof,2,2'-azobis(2-amidinopropane) and salts thereof; and4,4'-azobis-4-cyanovaleric acid, etc. These polymerization initiatorsmay be added at once or several times. Only one kind of the above-listedpolymerization initiators may be adopted, or two or more kinds thereofmay be suitably mixed and adopted. In the case of employing an oxidativeradical polymerization initiator, a redox polymerization may be carriedout with a combined use of a reducing agent such as sulfite, bisulfite,L-ascorbic acid, etc.

An amount of use of the polymerization initiators is preferably in arange of from 0.001 percent by weight to 5 percent by weight based onthe amount of the monomer component, more preferably in a range of from0.01 percent by weight to 1 percent by weight. It is not preferable touse the polymerization initiators in an amount less than 0.001 percentby weight, as effects of adopting the polymerization initiators would bepoor. It is also not preferable to use the polymerization initiator inan amount exceeding 5 percent by weight, as improved effects cannot beexpected from the effects achieved when adopting the polymerizationinitiators in the described range. Moreover, an average molecular weightof the resulting hydrophilic crosslinked polymer is reduced, whichcauses inferior shape retaining property. Here, the method ofpolymerizing (a) monomer component(s) is not particularly limited.

In order to obtain an absorbent material having desirable absorbingproperties, it is preferable that the hydrophilic crosslinked polymer isinner portion cross-linked by a reaction or a copolymerization reactionwith a cross-linking agent having a plurality of polymerizableunsaturated groups and/or a plurality of reactive groups. Namely, theresulting hydrophilic crosslinked polymer having a three-dimensional netstructure offers shape retaining property. The hydrophilic crosslinkedpolymer may be of self-cross-linkable type which does not require across-linking agent; however, it is preferable to adopt a cross-linkingagent.

The cross-linking agent is not particularly limited, and any compoundthat is reactive with an ethylenically unsaturated monomer or a polymerthereof may be used. Examples of such cross-linking agent include:tetraallyloxy ethane N,N'-methylene bis(meth)acrylamides, (poly)ethyleneglycol di (meth)acrylates, glycerol tri(meth)acrylates, trimethylolpropane tri(meth)acrylates, triallyl amine, triallyl cyanurate, triallylisocyanurate, glycidyl (meth)acrylate, (poly)ethylene glycol, diethyleneglycol, (poly)glycerol, propylene glycol, diethanol amine, trimethylolpropane, pentaerythritol, (poly)ethylene glycol diglycidyl ether,(poly)glycerol polyglycidyl ether, epichlorohydrin, ethylenediamine,polyethyleneimine, (poly)aluminum chloride, aluminum sulfate, potassiumchloride, and magnesium sulfate, etc. In consideration of itsreactivity, during or after the polymerization reaction, only one kindof the above-listed cross-linking agents may be adopted, or two or morekinds thereof may be suitably mixed and adopted. Among the above-listedcross-linking agents, it is preferable to adopt a cross-linking agenthaving a plurality of polymerizable groups, and among them, it is morepreferable to adopt a mixture of the cross-linking agent of at least onekind selected from the group consisting of triallylamine, tetraallyloxyethane, N,N'-methylenebis(meth)acrylamides, (poly)ethylene glycoldi(meth)acrylates, and trimethylol propane tri(meth)acrylate and amonomer component.

The amount of use of the cross-linking agent is generally in a range offrom 0.001 mole percent to 2 mole percent, preferably from 0.01 molepercent to 1 mole percent, based on the amount of the monomer component.It is not preferable to-use the cross-linking agent in an amount lessthan 0.001 mole percent, as the gel collapses when being compressedirrespectively of whether or not the gel contains foams, and may nothave desirable absorbing properties. It is also not preferable to usethe cross-linking agent in an amount above 2 mole percent,irrespectively of whether or not the gel contains foams, as it isdifficult to decompress the gel, and the resulting absorbent materialmay not have desirable absorbing properties.

Furthermore, the monomer component may be polymerized in a presence of ahydrophilic polymer such as starch, cellulose, chitin, polyvinylalcohol, polyacrylic acid (salts), and crosslinked polymers thereof,polyethylene glycol, etc. As a result, a reaction of forming a graftbond, a complex, is carried out simultaneously when carrying out thepolymerization reaction of a monomer. Namely, a hydrophilic crosslinkedpolymer having a graft bond, a complex, etc., between the polymer of amonomer component and the hydrophilic polymer.

The method of polymerizing the monomer component is not particularlylimited, and known polymerization methods such as a solutionpolymerization or reversed-phase suspension polymerization may beadopted. Examples of such solution polymerizations include but are notlimited to: a solution polymerization method in which an aqueoussolution of monomer components are placed in a predetermined mold; asolution polymerization method in which a mixer such as a kneader, etc.,equipped with a mixing blade of a predetermined shape is used as apolymerization device, and the resulting hydrophilic polymer gel ispulverized by a shear stress of the mixing blade, etc. Between thedescribed polymerization methods, the latter method is more preferableas a granular gel can be obtained.

When polymerizing the monomer components by aqueous solutionpolymerization, the solution of the monomer component may or may not bestirred. However, when carrying out a polymerization reaction in apresence of a foaming agent, it is preferable that the aqueous solutionof the monomer component be let still for at least a predetermined timeduring the reaction to achieve an effective forming by a foaming agent.Specifically, by leaving the aqueous solution of the monomer componentstill for a predetermined time from a start of polymerization reactiontill a point a polymerization rate becomes 10 percent, more preferably30 percent, still more preferably 50 percent, and the most preferablytill the completion of the reaction, a still improved foaming effect bythe foaming agent can be achieved. The time from the dispersion of thefoaming agent till the start of the polymerization reaction of themonomer component is not particularly limited. However, the shorter thetime, the more preferable it is.

When the gel of the hydrophilic cross-linked polymer resulting from theaqueous polymerization method is in a form of a bulk, it is preferableto pulverize the gel into particles having a predetermined diameter. Themethod of pulverizing the gel in a particular form is not particularlylimited, and known pulverization methods of applying a shear stress tothe gel may be adopted. Examples of the device of applying such shearstress include but are not limited to: screw type extruding devices likea meat chopper, various cutters, a (mechanical pressurizing) kneader, aninternal mixer, a Banbury mixer, and other similar kneaders.

As an example of the reversed-phase suspension polymerization method,for example, a method of performing a polymerization by suspending asolution of a monomer component in the presence of a dispersant in ahydrophobic organic solution may be adopted. The describedreversed-phase suspension polymerization enables a water-containing gelthat is spherical (particular) in shape or a cellular gel to be obtainedwithout pulverization upon completion of the polymerization reaction.

The hydrophilic crosslinked polymer is swollen by absorbing an aqueoussolvent, and when the ratio of the aqueous solvent based on a totalamount of hydrophilic crosslinked polymer and the aqueous solvent(hereinafter referred to as a water content) becomes not less thanaround 30 percent by weight, the hydrophilic crosslinked polymer isgelled. Namely, the water-containing gel of the hydrophilic crosslinkedpolymer and the cellular gel of the hydrophilic crosslinked polymergenerally have a water content of not less than around 30 percent byweight, preferably from 30 percent by weight to 90 percent by weight andstill more preferably from 40 percent by weight to 80 percent by weight.The water content may be adjusted based on the monomer density before orafter polymerization when an occasion demands, or may be adjusted by adrying treatment or post addition of the aqueous solvent.

When the water content is less than around 30 percent by weight, as thehydrophilic crosslinked polymer is not formed into the gel, it isdifficult to calendar the hydrophilic crosslinked polymer. Namely, inthe case of adopting the rigid hydrophilic crosslinked polymer, whencalendaring the cross-linked absorbent polymer, it is difficult tosufficiently compress the absorbent resin, i.e., each particle of thehydrophilic crosslinked polymer, to have a flat shape.

On the other hand, when the water content is above 90 percent by weight,irrespectively of whether or not the gel contains cells, the conveniencein handing the gel is lowered, and moreover, it is difficult to calendarthe gel. In such case, the gel strength becomes poor, and whencompressing by reducing the water content, particles of the hydrophiliccrosslinked polymer are not distorted but simply collapse. Therefore, itmay not be possible to retain the original shape of the absorbent resinby absorbing water. Namely, it becomes difficult to ensure a desirableshape retaining property. Therefore, such condition is unpreferable asthe desirable absorbing rate, and the absorbency under load may not beachieved. In the present invention, the shape retaining propertyindicates that the hydrophilic crosslinked polymer particles (absorbentresin) are immobilized in a distorted state, and when swollen byabsorbing water, the polymer particles are retained to an original statebefore a pressure was applied thereto (compression) by being swollen ina non-similar shape.

Here, it is obvious that the absorbent resin is swollen (anisotropicallyswells) into a non-similar shape from a comparison between the shape ofthe absorbent resin after compression and the shape of the absorbentresin after being swollen by absorbing water.

The described gel of the hydrophilic crosslinked polymer further absorbswater; body fluids such as urine, sweat, blood; an aqueous liquid suchas drip (juice) exuded from meats, fishes, vegetables, fruits; etc. Whencontacting the aqueous fluids, the gel is swollen by absorbing theaqueous fluids, which causes a further volume expansion. It ispreferable that the gel of the hydrophilic crosslinked polymer iscapable of absorbing the aqueous fluid of not less than three times ofthe dead weight (weight of the gel).

Further, the cellular gel of the hydrophilic crosslinked polymer shows avolume swell of from 1.01 to 10 times of the non-cellularwater-containing gel, preferably from 1.05 to 5 times, and morepreferably from 1.1 to 3 times. By adopting the cellular gel of thehydrophilic crosslinked polymer, an absorbent resin which showsexcellent absorbing properties can be obtained.

It is further preferable to adopt a porous gel as the cellular gel inthe present invention. When polymerizing the monomer component in thepresence of the foaming agent, the foaming agent dissolved in themonomer solution is foamed by vaporization, decomposition, deposition orvolume swell, etc., and cells (space) are formed in the resultingpolymer (gel-like hydrophilic crosslinked polymer). As a result, theporous gel (cellular gel) containing many foams inside can be obtained.

In the present invention, porous indicates that the gel contains per 1cm³ of the gel (cellular gel) at least 10 cells, preferably not lessthan 100 cells, more preferably not less than 1,000 cells, and stillmore preferably not less than 10,000 cells. Here, whether the cells arecontinuous or independent does not matter.

When the number of cells is less than 10, or the volume swell is lessthan 1.01, the gel of the hydrophilic crosslinked polymer may not offersufficient effects of improving the absorbing rate or permeability toliquid. On the other hand, when the number of cells exceeds 1,000,000 orthe volume swell exceeds 10 times the non-cellular water-containing gel,the volume efficiency of the manufacturing device is lowered, and thecost is increased, and further it becomes difficult to restore thecompression.

The porosity diameter of the cellular gel can be obtained by theanalysis of an image of the cross-section of the cellular gel by anoptical microphotograph. Namely, by preparing a histogram showing adistribution of the porosity diameter of the cellular gel by an imageanalysis, and computing an average number of holes based on thehistogram, an average porosity diameter is computed. In this case, themagnification of the optical microphotograph is not particularlylimited; however, it is preferably in a range of from 10 times to 1,000times, and still more preferably in a range of from 20 times to 100times.

In consideration of the absorbing rate and the permeability to aqueousliquid, the average porosity diameter of the cellular gel is generallyin a range of from 1 μm to 1,000 μm, preferably in a range of from 10 μmto 300 μm, and more preferably in a range of from 20 μm to 200 μm. Byadjusting the porosity diameter of the cellular gel within the describedrange, an absorbent material which shows excellent absorbing rate andpermeability to aqueous liquid can be obtained.

Furthermore, as the cellular gel is porous having an average porositydiameter in the described range, a sufficient space for passing theaqueous liquid inside the cellular gel, i.e., the absorbent resin, canbe ensured both without an applied pressure and under an appliedpressure. Therefore, the cellular gel shows excellent permeability toaqueous liquid and dispersibility and improved absorbing rate and waterretention characteristics can be achieved by the capillarity.

It is preferable that the water-containing gel and the cellular gel ofthe present invention are in a granular form. Namely, the respectiveshapes of the particles, i.e., the absorbent resin when absorbing waterare not particularly limited, and may be formed into cubic, polyhedron,spherical, disk, sector, stick-like, needle, fabric, flake or undefined(irregular) shape, etc. The water-containing gel or the cellular gel maybe primary particles or agglomerates (secondary particles) in whichprimary particles agglomerate. Among these shapes, those of irregularshapes obtained in the process of pulverizing in which particlesdiameters are not uniform, or spherical shape obtained by a reversephase suspension polymerization are especially preferable.

The described water-containing gel and the cellular gel have a wideparticle size distribution; however, it is preferable to have apredetermined range of particle size distribution and a predeterminedaverage particle diameter. It is preferable that the average particlediameter of the resulting water-containing gel and the cellular geldried, i.e., the average particle diameter of the hydrophiliccrosslinked polymer resulting from respective polymerization reactions(hereinafter, referred to as a dried average particle diameter) be in arange of from 50 μm to 2,000 μm, more preferably in a range of from 60μm to 1,500 μm, still more preferably in a range of from 80 μm to 1,000μm, and most preferably in a range of from 100 μm to 600 μm.

In order to achieve an absorbent material and an absorbent article whichare soft and comfortable to use and convenient to handle, it ispreferable that the water-containing gel and the cellular gel do notsubstantially contain particles having a particle diameter of not lessthan 5 mm, and more preferably do not substantially contain particleshaving a particle diameter of not less than 3 mm.

The gel having a dried average diameter of above 2,000 μm is inferior inits handling, surface smoothness, and moreover, a surface area per unitweight of the gel is relatively small although the gel has foamsirrespectively of whether or not the gel is cellular. Therefore, it isnot preferable as the absorbing rate of the resulting absorbent materialis low. Moreover, it is not preferable to adopt the gel having a driedaverage particle diameter of less than 50 μm, irrespectively of whetheror not the gel contains foams, as the convenience in handling the geland the permeability to aqueous liquid suffer.

The dried average particle diameter can be converted after classifyingthe gel in the following manner. Namely, first, the beaker of apredetermined size (container) is placed on a magnetic stirrer, and1,200 g of an aqueous solution of 20 percent by weight of sodiumchloride is placed therein. Then, after placing 25 g of the target gelhaving a solid portion of α percent by weight, the gel is dispersed byrotating the rotator at 300 rpm. After stirring it for 60 minutes, thedispersed solution is poured into six sieves stacked in the order of thesieve openings (from the top sieve). Here, the respective sieve openingsare selected to be r₁ =0.075 mm, r₂ =0.30 mm, r₃ =0.60 mm, r₄ =0.85 mm,r₅ =2.0 mm, and r₆ =9.5 mm from the bottom. Then, 6,000 g of 20 percentby weight of sodium chloride are poured gently from the top, therebyclassifying the gel.

After fully rinsing off the gel thus classified, the weight of thewater-containing gel is measured. Here, the sum of the weight ofrespective gels, i.e., the total weight of the gels thus classified andrinsed off is determined to be W(g). Then, according to the followingformula R_(n) =[(α/100)·(25/W)]^(1/3) ×r_(n), the opening r_(n) isconverted into the opening R_(n) (mm) in which the dried gel isclassified. The weight (percent by weight) of the gel remaining on eachsieve with respect to the total weight W is measured.

Then, each respective ratio of the gel remaining on the sieve R_(n) tothe gel remaining on the sieve respectively having an opening of R_(n)(R₁, R₂, R₃, R₄, R₅, R₆) is plotted on an algorithmic probability paper.From the plotted graph, the opening R at which the ratio of the weightof the gel with respect to the total weight W is 50 percent is read, andthis opening R is determined to be an average particle diameter (mm) ofthe dried gel, thereby obtaining the dried average particle diameter.

It is further preferable that the water-containing gel and the cellulargel respectively have a water-soluble component of not more than 20percent by weight, preferably in a range of from 0.1 percent by weightto 20 percent by weight, and still more preferably in a range of from 1percent by weight to 15 percent by weight. It is not preferable toselect the content of the water-soluble component to be above 20 percentby weight, as sufficient gel strength cannot be achieved. On the otherhand, if the content of the water-soluble component is less than 0.1percent by weight, the absorbency and the absorbing rate of theabsorbent material become insufficient.

It is further preferable that the conversion of the gel falls in a rangeof from 90 percent to 99.99 percent irrespectively of whether nor notthe gel contains foams. If the polymerization rate of the gel is lessthan 90 percent, the properties will be degraded as the water content isreduced, and the described shape retaining property may not be ensured.

In the present invention, by specifying the water content or thewater-soluble component content of the gel, the kind of the main chainof the hydrophilic cross-linked polymer, the average particle diameteror the dried average particle diameter of the gel, a still improvedshape retaining property can be obtained.

When preparing the absorbent material in accordance with the presentinvention, it is preferable that these gels (water-containing gel, thecellular gel) further contain polyhydric alcohol. Examples of suchpolyhydric alcohol include but are not limited to: ethylene glycol,diethylene glycol, polyethylene glycol, triethylene glycol,tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropyleneglycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol,polyglycerol, 2-butene-1,4-diol, 1,4-butane diol, 1,5-pentane-diol,1,6-hexanediol, 1,2-cyclohexane dimethanol, 1,2-cyclo-hexanol,trimethylolpropane, diethanolamine, triethanol amine, polyoxy propylene,oxyethylene-oxypropylene block copolymer, pentaerythritol, sorbitol,polyvinyl alcohol, glucose, mannitol, sucrose, dextrose, etc. Only onekind of the above-listed polyhydric alcohol may be adopted, or two ormore kinds thereof may be suitably mixed and adopted. Among theabove-listed polyhydric alcohol, glycerol is the most preferable.

The gel including polyhydric alcohol offers an absorbent material thatcan be formed into a sheet with ease and shows excellent flexibility,strength and cushioning characteristics. When the absorbent materialcontacts water, the absorbent resin, i.e., the hydrophilic crosslinkedpolymer particles can be restored into an original shape before having apressure applied thereto. Namely, an improved shape retaining propertycan be achieved.

The (total) amount (hereinafter referred to as a total amount) of thepolyhydric alcohol with respect to a total amount of a solid portion anda polyhydric alcohol portion of the gel (water-containing gel or thecellular gel) is preferably in a range of from 0.1 percent by weight to80 percent by weight, more preferably in a range of from 1 percent byweight to 60 percent by weight, and still more preferably in a range offrom 5 percent by weight to 30 percent by weight. By using thepolyhydric alcohol in the described range, a gel can be formed into asheet with ease, and the gel can be easily pulverized. Moreover, theflexibility and the strength of the absorbent material (tensilestrength, tearing strength) are improved. It is not preferable to setthe ratio of the polyhydric alcohol based on a total amount to be lessthan 0.1 percent by weight, because the effect achieved by adopting thepolyhydric alcohol is small, and a sufficient strength cannot be appliedto the absorbent material. On the other hand, it is also not preferableto set the ratio of the polyhydric alcohol to larger than 80 percent byweight with respect to the total amount, as too much polyvalent alcoholis used, which causes the absorbent material to be sticky and lowersvarious absorbing properties of the absorbent material. In the presentinvention, the method of mixing the gel of the hydrophilic crosslinkedpolymer and the polyhydric alcohol, i.e., the method of preparing amixture of the water-containing gel or the cellular gel with polyvalentalcohol (hereinafter simply referred to as a mixture) is notparticularly limited.

When pressurizing the hydrophilic crosslinked polymer gel, thehydrophilic crosslinked polymer may be surface-crosslinked (secondarycrosslinkage) by further adding a surface cross-linking agent. Thesurface cross-linking agent is not particularly limited, and any knownsurface cross-linking agent of a compound having a plurality of reactivegroups and which is reactive to a functional group such as a carboxylgroup of the hydrophilic crosslinked polymer may be used.

Examples of such surface cross-linking agent include but are not limitedto: polyhydric alcohol such as ethylene glycol, diethylene glycol,propylene glycol, triethylene glycol, tetraethylene glycol, polyethyleneglycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol,polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol,trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene,oxyethylene-oxypropylene block copolymer, pentaerythritol, sorbitol,polyvinyl alcohol, glucose, mannitol, sucrose, dextrose, etc.;polyhydric epoxy compounds such as ethylene glycol diglycidyl ether,polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether,diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether,propylene glycol diglycidyl ether, polypropylene glycol diglycidylether, etc.; polyhydric amine compounds such as ethylenediamine,diethylenetriamine, triethylenetetramine, tetraethylenepentamine,pentaethylenehexamine, polyethyleneimine, etc.; polyhydric isocyanatecompounds such as 2,4-tolylene diisocyanate, hexamethylene diisocyanate,etc.; polyhydric oxazoline compounds such as 1,2-ethylene bisoxazoline,etc.; alkylene carbonate compounds such as 1,3-dioxolane-2-one,4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one,4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one,4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one,4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one,1,3-dioxopane-2-one, etc.; haloepoxy compounds such as epichlorohydrin,epibromohydrin, α-methylepichlorohydrin, etc.; polyvalent metalliccompounds such as hydroxides and chlorides of metals: zinc, calcium,magnesium, aluminum, iron, zirconium, etc. Only one kind of theabove-listed surface cross-linking agent may be adopted, or two or morekinds thereof may be suitably mixed and adopted. Among the above-listedsurface cross-linking agents, polyvalent epoxy compound is preferable.

As described, by surface cross-linking the hydrophilic polymer using thesurface cross-linking agent, the shape retaining property, and theabsorbency under pressure of the absorbent material are still improved.Moreover, the dispersibility of the aqueous liquid and various absorbingproperties such as permeability, etc., when absorbing the aqueous liquidcan be improved. Furthermore, when contacting the aqueous liquid, acomponent to be eluted in the aqueous liquid, i.e., an amount of thewater-soluble component can be reduced.

The amount of use of the surface cross-linking agent may be suitablyselected according to the kind of the surface cross-linking agent, acombination thereof, a degree of surface cross-linkage, etc. However, anamount of use of the surface cross-linking agent is generally in a rangeof from 0 to 10 percent by weight, preferably in a range of from 0.001percent by weight to 5 percent by weight, and still more preferably in arange of from 0.01 percent by weight to 1 percent by weight.

Additionally, a mixture of the water-containing gel, the cellular gel, amixture of each gel and polyhydric alcohol and a surface cross-linkingagent is not particularly limited. When applying a pressure to the gel,by applying pressure and at the same time heat, a crosslinkable reactionof the hydrophilic cross-linked polymer and the surface cross-linkingagent can be further accelerated.

When applying a pressure to the gel, preferably when calendaring thegel, if necessary, an auxiliary molding compound may be used. Namely,the absorbent material may include an auxiliary molding compound otherthan gel and polyhydric alcohol. As the molding compound, a surfaceactive agent, fibers, various fine water-insoluble particles, etc., maybe used. Such auxiliary molding compounds may be used alone or incombination. When forming the absorbent material into a sheet, it ispreferable that the absorbent material contains fibers.

The described surface active agent includes anionic surface activeagents, nonionic surface active agents, cationic surface active agents,and amphoteric ionic surface active agents. Examples of anionic surfaceactive agents include but are not limited to: fatty acid salts of sodiumoleate, potassium castor oil, etc., alkylsulfuric ester salts of laurylsodium sulfide, lauryl ammonium sulfide, etc., alkylbenzene sulfonicacid salts such as dodecyl benzene sodium sulfonic acid salts, etc.,alkyl naphthalene sulfonic acid salts, dialkyl sulfo-succinate, alkylphosphate salts, and naphthalenesulfonic formalized condensationproduct, and polyoxyethylene alkyl sulfate salts, or the like.

Examples of the nonionic surface active agent include but are notlimited to: polyoxyethylene alkyl ether, polyoxyethylene alkyl phenolether, polyoxyethylene fatty acid ester, sorbitan fatty acid ester,polyoxyethylene sorbitan fatty acid ester, polyoxyethylene alkylamine,and an oxyethylene-oxypropylene block copolymer.

Examples of the cationic surface active agent include but are notlimited to: alkyl amine salts such as lauryl amine acetate, stearylamine acetate, etc., quaternary ammonium salts such as lauryl trimethylammonium chloride, stearyl trimethyl ammonium chloride, or the like.

Examples of the amphoteric ionic surface active agent include but arenot limited to: lauryl dimethylamine oxide. Only one kind of theabove-listed surface active agents may be adopted, or two or more kindsthereof may be suitably mixed and adopted. By adopting the surfaceactive agent, the resulting mixture can be calendared to be formed intoa sheet still more easily.

For the described fibers, both long fiber and short fiber may beadopted. Examples of such fiber include but are not limited to: woodfibers such as pulp, etc., natural fibers such as hemp, etc., polyester,inorganic fibers such as a glass fiber, etc. Examples of polyesterinclude but are not limited to polyethylene terephthalate (PET), etc.Only one kind of the above-listed fibers may be adopted, or two or morekinds thereof may be suitably mixed and adopted. Among the above-listedfibers, synthetic fibers are preferable and hydrophobic synthetic fibersare still more preferable. Besides, paper (Japanese paper) made fromthese fibers, thread, woven or non-woven cloth, etc., may be adopted. Byadopting the fiber, a still improved absorbing rate of the resultingabsorbent material can be obtained, and a still improved shape retainingproperty can be achieved. Furthermore, when forming the absorbentmaterial into a sheet, the sheet can be made thin uniformly (forexample, several mm, etc.,). When kneading the water-containing gel, thecellular gel or a mixture of each gel and polyhydric alcohol with thefibers by the kneader, it is preferable to adopt short fibers of from 2mm to 50 mm length, preferably from 10 mm to 40 mm length, and stillmore preferably from 20 mm to 30 mm length so as to prevent a mixingblade of the kneader from being caught by fibers.

Examples of minute particles include but are not limited to: mica,pyrophyllite, kaolinite, hulsite, and inorganic material similar toclayish minerals, and silica (silicon dioxide) having an averageparticle diameter of not more than 50 μm, such as Aerosil 200 (availablefrom Japan Aerosil Ltd.), Carplex #80 (available from Shionogi & Co.,Ltd.) may be cited; carbon black, activated carbon, etc. Only one kindof the above-listed minute particles may be adopted, or two or morekinds thereof may be suitably mixed and adopted.

The amount of use of the auxiliaries is not particularly limited, andmay be suitably adjusted in consideration of the kind or combination,etc., thereof. The total amount of the auxiliaries is generally in arange of from 0.01 to 100 parts by weight, desirably from 0.1 to 50parts by weight, and more desirably from 0.1 to 30 parts by weight,based on 100 parts by weight of the solid portion of the gel (i.e.,hydrophilic crosslinked polymer). When the auxiliaries are used in anamount above 100 parts by weight, the resulting absorbent material islikely to be hardened. The method of mixing the water containing gel,the cellular gel, or a mixture of each gel and the polyhydric alcoholand the mixing conditions are not particularly limited.

According to the present invention, by reducing the water content of thewater-containing gel under an applied pressure, an absorbent material(I) containing a non-cellular gel which is swollen by absorbing water soas to have anisotropy can be obtained. The present invention alsopermits the absorbent material (II) containing a porous absorbent resinwhich is swollen by absorbing water so as to have anisotropy to beobtained by pressurizing the cellular gel while reducing water contentif necessary. In the present invention, when adopting the gel havingfoams inside, i.e., the cellular gel, as the gel, depending on the watercontent, it is not necessary to always apply pressure and reduce watercontent at the same time. However, in order to optimize the effect ofthe shape retaining property, it is preferable to apply pressure whilereducing the water content even when adopting the cellular gel as thegel. As the absorbent resin is formed into a shape before having appliedpressure (compression) by the anisotropic expansion of the absorbentresin in a non-similar shape, an absorbent material ((absorbent material(I) or absorbent material (II)) which has still improved absorbingproperties such as absorbing rate, absorbency under load and shaperetaining property can be obtained.

The method of pressurizing the gel is not particularly limited, and theknown method may be adopted. For the pressurizers to be adopted in thepresent invention, a device of reducing the water content from the gelunder an applied pressure (amount of aqueous solvent) is preferable, anda device of molding into a sheet by calendaring is preferable. Thedescribed pressurizers include but are not limited to a drum dryer,etc., provided with a combination of a compressor. When pressurizing thegel, by compression forming the absorbent material (absorbent material(I) or absorbent material (II)) by reducing the water content of the gelwhile applying pressure, an absorbent material for retaining theoriginal shape before having an applied pressure can be obtained.

Examples of the method of reducing water content include but are notlimited to (i) a method of reducing a water content by reducing pressureat a temperature below room temperature while compressing the gel; (ii)a method of removing water by immersing the gel in a hydrophilic organicsolvent under an applied pressure; and (iii) a method of heating the gelunder an applied pressure, etc., and among the above-listed threemethod, the method (iii) is the most preferable.

In this case, examples of the heating method include but are not limitedto: 1 a heating method of a conductive heat transfer type for heating ofa gel by making the gel in direct contact with a heating surface of theheater; 2 a heating method of a hot air heat transfer type by a hot airor steam, etc.; 3 a radiant heating method of a transfer-type byradiation such as an infrared ray, an extreme infrared ray, etc., 4dielectric heating by microwaves, etc. These methods can be selectivelyused. Among all, 1 a heating method of conductive heat transfer systemby a heat plate, heat drum, a heat roller, a heat belt, etc., ispreferable. It is especially preferable to adopt a method of applyingheat and pressure to a heating surface of the absorbent material(absorbent material (I) or the absorbent material (II)), i.e., a heatingsurface of the gel by combined use of a plate, a drum, a roller, a belt,etc.

When adopting the method 1, it may be arranged so as to generate adistortion in the cross-linked structure of the absorbent resin byapplying heat and pressure to the heating face of the gel so that atemperature difference occurs, for example, between the heating surface(first face) that is a contact face with the heater and the backsurface, i.e., an opposite surface (second surface) to the first surfacein a gel thickness direction to generate a distortion in thecross-linked structure of the absorbent resin. When obtaining asheet-like absorbent material (I)•(II) (sheet) by the above method, asthe shape retaining property of the absorbent resin (shape retainingproperty) of the absorbent resin differs between the first face (namely,heating surface) and the second face (for example, non-heating surfaceor low temperature surface) of the sheet, the sheet can be curled so asto have a curvature to have a low temperature region inside. The sheetabsorbs physiologic saline solution from 1 to 50 times, preferably from2 to 25 times, and more preferably from 5 to 20 times by weight of thehydrophilic crosslinked polymer, so that both ends of the sheet can becurled. This phenomenon occurs irrespectively of the size of the sheet.However, the phenomenon can be observed with ease by making a from 1 cmto 10 cm sheet be swollen by absorbing water.

Next, referring to FIG. 1, a method of pressurizing a gel (gelcomposition) by adopting a drum dryer provided with a compressor as oneexample of a calendaring method will be explained in reference toFIG. 1. The pressurizing method of the gel is not limited to the methodof adopting the drum dryer provided with the compressor.

The drum dryer is, for example, an open-type single drum dryer, and asshown in FIG. 1, the drum dryer (single drum dryer) provided with thecompressor includes a drum dryer 1, a pressurizing roller 2 as acompressor, a scraper 3, and a driving unit (not shown), etc. The drumdryer 1 is made of stainless, etc., and is rotatably driven in adirection shown by an arrow A at number of rotations of, for example,not more than several rpm. In the dryer drum 1, a heater (not shown) isstored so as to permit the surface of the drum dryer 1 to be heated to apredetermined temperature.

The pressurizing roller 2 is made of a stainless, etc., and apredetermined interval is formed from the surface of the drum dryer 1.Namely, a predetermined clearance is formed between the pressurizingroller 2 and the surface of the drum dryer 1. The pressurizing roller 2is rotatably driven in a direction of an arrow B at a predeterminednumber of rotations with respect to the number of rotations of the drumdryer 1. Then, the pressurizing roller 2 is able to apply apredetermined pressure with respect to a gel composition 10 (to bedescribed later) as a mixture that is fed on the surface of the drumdryer 1. Namely, the drum dryer 1 is capable of reducing the watercontent of the gel while applying pressure to a gel composition 10.

The scraper 3 is formed so as to be in contact with the surface of thedryer drum 1, and a sheet 11 (to be described layer) adhering to thesurface is scraped off. On the surface of the drum dryer 1 and thesurface of the pressure roller, mirrors are formed. In the pressurizingroller 2, a heater may be stored so as to heat the surface of thepressurizing roller 2 to a predetermined temperature.

In the described arrangement, on the upstream side of the pressurizingroller 2 attached to the drum dryer 1, a gel composition 10 composed ofthe gel (water-containing gel or cellular gel), polyhydric alcohol and,if necessary, a subsidiary foaming material are fed. The gel composition10 adhering to the surface of the drum dryer 1 is transported in thedirection of an arrow A, to reach a clearance between the drum dryer 1and the pressurizing roller 2.

Then, by the pressurizing roller 2, the gel composition 10 ispressurized (calendared) to reduce the thickness to not more than 15percent, preferably not more than 10 percent and still more preferablynot more than 5 percent of the original thickness to form a sheet 11.Then, the sheet 11 being transported in the direction of an arrow Aadhering to the surface of the drum dryer 1 is heated by the aboveheater via the surface of the drum dryer 1. The temperature of thesurface of the drum dryer 1, i.e., the heating temperature of the gelcomposition 10 is preferably in a range of from room temperature to 300°C., more preferably in a range of from 50° C. to 200° C., and still morepreferably in a range of from 100° C. and 180° C. It is not preferableto set the heating temperature above 300° C., as the hydrophiliccrosslinked polymer and the polyvalent alcohol decompose. By applyingheat to the sheet 11, namely, by heating the gel contained in the gelcomposition 10, the water content (amount of aqueous solvent) isreduced. Namely, with an application of heat, a part of polyhydricalcohol may be evaporated.

The reduction ratio of the aqueous solvent in the gel is notparticularly limited. Although the reduction ratio differs depending onwhether or not the gel contains foams, it is preferably in a range offrom 10 percent by weight to 90 percent by weight, and more preferablyin a range of from 40 percent by weight to 80 percent by weight based onthe amount of the aqueous solvent before applying pressure.

When the gel composition 10 is not heated, i.e., when the temperature onthe surface of the drum dryer 1 is at room temperature, the ratio ofreduction in water content is substantially 0 percent by weight. If theratio of reduction in water content is small, it is difficult to obtainthe desirable shape retaining property. Additionally, when reducing thewater completely (the gel having a water content of 0%), it is likelythat various properties and flexibility are degraded.

When pressurizing the gel composition 10, in order to obtain a desirableshape retaining property, it is preferable to pressurize the gel so asnot to be subdivided (collapse). For this reason, it is preferable thatthe gel (water-containing gel, cellular gel) is sub-divided to have thedescribed fine particle diameters beforehand.

After being transported in a direction of an arrow A, the sheet 11 isscraped off from the surface of the drum dryer 1 by the scraper 3. Thesheet 11 thus scraped off, i.e., the absorbent material is wound arounda roll, etc., (not shown) when the need arises. Additionally, as theabsorbent material has an appropriate flexibility or strength byincluding the polyhydric alcohol, it would not be broken by being bent,for example, at 90° or further at 180° when being removed by the scraper3. Alternatively, the sheet 11 may be separated from the surface of thedrum dryer 1 by applying an appropriate tension to the absorbentmaterial. As a result, a sheet-like absorbent material (absorbentmaterial (I) or absorbent material (II)) having at least one smoothsurface can be obtained.

As described, according to the present invention, by reducing the watercontent while applying pressure to the water-containing gel, theabsorbent material (I) having an excellent water absorbing rate andabsorbency under load can be obtained. As described, in the presentinvention, by applying a pressure to the cellular gel, preferably byapplying pressure while reducing water, the absorbent material (II)which is superior to the absorbent material (I) in its absorbing rateand absorbency under load can be obtained.

Generally, after obtaining a powdery absorbent material, when forming,for example, a sheet-like powdery absorbent material, the absorbing rateand absorbency under load of the resulting absorbent material aresignificantly lowered by comparing with the pre-formed absorbentmaterial. For this reason, it is difficult to prepare an absorbentmaterial having excellent absorbency under load and absorbing ratesuited for use in molding articles by the conventional method. However,according to the present invention, it is permitted to directly form(into a sheet) from the water-containing gel or cellular gel, therebyobtaining a molded article of high value (for example, sheet-likeabsorbent materials (I) and (II)).

According to the present invention, the absorbing rate of the absorbentmaterial (I) as a molding article (sheet) adopting the water containinggel is not more than 150 seconds, and with a combined use of the surfacecross-linking agent, or by specifying the water content, water-solublecomponent in the water-containing gel, the kind of a main chain of thehydrophilic crosslinked polymer, the average particle diameter or thedried average particle diameter of the water-containing gel, a highabsorbing rate of not more than 100 seconds, more preferably not morethan 50 seconds and still more preferably not more than 30 seconds canbe achieved. However, it is not preferable to set the absorbing rate tobe not more than 1 second, particularly not more than 0.5 seconds, asthe absorbing rate is too low which causes the liquid diffusivity to belowered.

In the present invention, the absorbing rate of the absorbent material(I) as a molding (sheet) in the case of adopting water-containing gel asthe gel, particularly, the absorbing rate under applied pressure is notless than 15 g/g, and with a combined use of the surface cross-linkingagent, the water content and water-soluble content in thewater-containing gel, the kind of a main chain of the hydrophilicpolymer, the average particle diameter, or the dried average particlediameter, etc., of the water-containing gel, an absorbency of preferablynot less than 20 g/g and more preferably not less than 25 g/g can beobtained.

On the other hand, in the present invention, absorbing rate of theabsorbent material (II) as a molding (sheet) in the case of adoptingcellular gel as the gel is not more than 150 seconds, and with acombined use of the surface cross-linking agent, or by specifying theamount of water or the water-soluble component in the cellular gel, theporosity diameter, or number of foams in unit volume, the kind of mainchains of the hydrophilic crosslinked polymer, the average particlediameter, or the dried average diameter of cellular gel, the absorbingrate can be set to not more than 100 seconds, more preferably not morethan 50 seconds, and still more preferably not more than 30 seconds, andthe most preferably not more than 20 seconds. However, when theabsorbing rate is not more than 1 second, particularly not more than 0.5seconds, the absorbing rate is too high which causes the liquiddiffusivity to be lowered.

In the present invention, in the case of adopting cellular gel as thegel, the absorbency of the absorbent material (II) as a molding (sheet),particularly the absorbency under applied pressure is not less than 15g/g, and with a combined use of the surface cross-linking agent or byspecifying the water content or the water-soluble content in thecellular gel, the porosity diameter, or the number of foams in the unitvolume, the kind of a main chain of the hydrophilic crosslinked polymer,the average particle diameter or the dried average particle diameter ofthe cellular gel, an absorbency of not less than 20 g/g, more preferablynot less than 25 g/g, and more preferably not less than 30 g/g can beobtained. As described, by adopting the cellular gel as the gel, anabsorbent material (II), for example, formed into a sheet havingexcellent absorbing properties can be obtained such as an absorbing rateof not more than 20 seconds and an absorbency under load of not morethan 30 g/g.

Conventionally, when forming powdery absorbent resin (for example, in asheet form), etc., the absorbency under load and the absorbing rate ofthe resulting absorbent material are significantly reduced. However, inthe present invention, a high value molding particle (absorbentmaterial) can be obtained.

As described, according to the present invention, by pressurizing thewater-containing gel or cellular gel, and reducing a water content inthe water-containing gel or cellular gel, absorbent material (absorbentmaterial (I) or absorbent material (II)) having excellent absorbingproperties particularly in absorbing rate and absorbency under load canbe obtained compared with conventional absorbent material.

Namely, when pressurizing the water-containing gel of the hydrophiliccrosslinked polymer or cellular gel of the resulting hydrophilicpolymer, by removing the aqueous solvent from the three-dimensional netstructure of the water-containing gel or the cellular gel, thethree-dimensional net structure is distorted under an applied pressure.

Therefore, for example, by pressurizing the gel composition 10containing the water-containing gel as the gel, as shown in FIG. 2(a),the water content of the non-cellular absorbent resin 30 as across-linked gel-like hydrophilic polymer particles is reduced, and isgreatly compressed in a direction of applying pressure, while beingcalendered in a direction of expanding in a perpendicular direction withrespect to a pressurizing direction.

As shown in FIG. 3(a), by pressurizing the gel composition 10 containingcellular gel as the gel, cellular gel particles, i.e., the absorbentresin 31 having foams 31a inside as gel-like hydrophilic polymerparticles are compressed in a large degree in a pressurizing directionfor a reduction in amount of water content, while being calendered in adirection perpendicular to a pressurizing direction.

As a result, these absorbent resins 30 and 31 are compressed andimmobilized in oblately distorted state to have a compression ratio ofnot less than 2, preferably in a range of from 5 to 1,000, morepreferably in a range of from 10 to 200, and still more preferably in arange of from 15 to 100.

According to the present invention, the compression ratio suggests theexpansion ratio in the direction of calendering the absorbent resin withrespect to the compression ratio in the direction of compressing theabsorbent resin.

Therefore, the compression ratio of respective absorbent resins 30 and31 can be measured by the following method. Here, as the absorbent resin30, the primary water containing gel may be used. Additionally, as theabsorbent resin 31, the primary particle of the cellular gel may beused.

In the case of adopting the compression ratio of the absorbent resin 30,first, an average particle diameter of the absorbent resin 30 isobtained. Next, the thickness of the absorbent resin 30 after an appliedpressure in the compressing direction and the average length (diameter)D₂ in a calendering direction are measured in a unit 0.01 mm, forexample, by a slide caliper. Then, the compression ratio X is obtainedby dividing the thickness D₁ in the compressing direction of theabsorbent resin 30 under an applied pressure by an average particlediameter of the absorbent resin 30. Then, an expansion ratio Y isobtained by dividing the average length (diameter) D₂ in the drivingdirection of the absorbent resin after having an applied pressure by anaverage particle diameter of the resulting absorbent resin 30. In thedescribed operation, the measurement is made with respect to from 10 to100 particles, and a compression ratio of the absorbent resin 30 isobtained with an average value of Y/X.

The compression ratio of the absorbent resin 31 can be measured in thesame manner as that adopted in measuring the compression ratio of theabsorbent resin 30. Namely, first, the average particle diameter of theabsorbent resin 31 is obtained. Next, the thickness D₃ in thecompressing direction of the absorbent resin 31 after being pressurized,and the average length (diameter) D₄ in the calendering direction aremeasured in a unit of 0.01 mm, for example, by a slide caliper. Then, bydividing the thickness D₃ in the compressing direction of the absorbentresin 31 after having an applied pressure by the average particlediameter of the absorbent resin 31, a compression ratio X is obtained.Similarly, by dividing the average length (diameter) D₄ in thecalendering direction of the absorbent resin 31 under an appliedpressure with an average particle diameter of the resulting absorbentresin 30, the expansion ratio Y is obtained. In the described operation,the measurement is made with respect to from 10 to 100 particles, andbased on the average value of Y/X, the compression ratio of theabsorbent resin 31 is obtained.

When the compression ratio is less than 2, the compressed absorbentresins 30 and 31 become bulky, and this makes it difficult to reduce thesize of the absorbent material and the size of the absorbent articlecontaining the absorbent material. When the compression ratio is lessthan 2, the distortion of the absorbent resin can be minimized, and itis difficult to ensure a significant improvement in the absorbing rate.Additionally, in the case where the compression ratio exceeds 1,000, theabsorbent resins 30 and 31 would be largely distorted, and the shapesare broken, and this makes it difficult to restore the original shape ofthe absorbent resin after being swollen by absorbing water.

As described, the absorbent resins 30 and 31 have the distortedcross-linked structure under an applied pressure (compression), and thuscompressed, immobilized (unmovable) in an oblately distorted state. As aresult, the absorbent resins 30 and 31 have distorted energy inside, andwhen contacting water, distorted energy is released, thereby generatingan internal stress of a size different in respective coordinate axes (x,y, z) against the distortions of the cross-linked structure.

Therefore, when contacting water, the absorbent resin 30 quickly absorbswater to retain its original shape (shown in FIG. 2(b)), and is swollenby quickly absorbing water so as to have an anisotropy by the internalstress.

Namely, from the original state in the gel form shown in FIG. 2(b), theabsorbent resin 30 is compressed to be restored in a stage beforeapplying pressure shown in FIG. 2(b). Then, the absorbent resin 30 isswollen by absorbing water, thereby restoring the original shape beforeapplying pressure. The absorbent resin 30 has the same shape before andafter applying pressure.

When contacting water, the absorbent resin 31 absorbs water quickly, andin order to restore the state before applying pressure (compressing)shown in FIG. 3(b), the absorbent resin 31 is swollen (anisotropicswelling) so as to have an anisotropy by the inner stress.

Namely, the absorbent resin 31 is formed into the compressed state shownin FIG. 3(a) under an applied pressure from the initial state of the gel(shown in FIG. 3(b)), and as a result of swelling by absorbing water, itis retained in the original state shown in FIG. 3(b). There is no changein the shape of the absorbent resin 31 before and after having apressure applied thereto.

Moreover, as shown in FIG. 3(a) and FIG. 3(b), as the absorbent resin 31has foams 30a, a BET specific area (when drying) of not less than 30times of the non-foamed absorbent resin 30 and has a sufficient liquidintroducing space required for moving the aqueous liquid inside.Therefore, as described, according to the present invention, when theabsorbing rate is not more than 20 seconds, and the absorbency underload is not less than 30 g/g, excellent absorbing properties can beobtained.

U.S. Pat. No. 4,920,202, U.S. Pat. No. 5,075,344, and U.S. Pat. No.5,145,906 disclose a method of drying the water-containing gel of thehydrophilic crosslinked polymer, and further by pulverizing, ifnecessary, to obtain absorbent resin powders.

However, by simply drying the gel, the gel shrinks in an equivalentdirection, the resulting absorbent resin is dried without beingdistorted and formed into powders. As a result, the resulting powderyabsorbent resin is swollen by absorbing water in a similar shape.

In contrast, in the present invention, by reducing the water contentwhile applying pressure, the distorted gel shrinks in the distortedstate to retain its original shape. For the shape retaining property, itis the essential conditions that the absorbent resin (absorbent resins30 and 31) has a cross-linked structure, and the state of the absorbentresin before an applied pressure is in a gel state, and it is impossibleto restore the shape for the polymer which does not have thecross-linked structure.

As described, according to the absorbent material (I) containing theabsorbent resin 30 and the absorbent material (II) of the presentinvention containing the absorbent resin 31 show excellent properties intheir absorbing rate and the absorbency under load and the shaperetaining property as the absorbent resins 30 and 31 retain theiroriginal shapes before having an applied pressure.

The water content of the water-containing absorbent materials (I) and(II) of the present invention is preferably in a range of from not morethan 80 percent by weight, more preferably in a range of from 5 percentby weight to 50 percent by weight, and more preferably in a range offrom 5 percent by weight to 30 percent by weight, and still morepreferably in a range of from 6 percent by weight to 25 percent byweight. The respective water content of the absorbent materials (I) and(II) may be adjusted, for example, by reducing the water content of thegel under an applied pressure. However, the water content may beadjusted after having an applied pressure if necessary by adding wateror drying.

The respective water content of the absorbent materials (I) and (II)indicate the ratio of the aqueous solvent based on the total amount ofthe aqueous solvent, the hydrophilic crosslinked polymer and thepolyhydric alcohol contained in respective absorbent materials (I) and(II). In the case where the absorbent materials (I) and (II) contain thesubsidiary forming material or other materials, when computing the watercontent, the subsidiary forming material or other forming materials arenot considered.

The respective methods of measuring the water (content), the absorbencyunder load and absorbing rate are described in detail under preferredembodiments of the present invention. It should be noted here that theabove-defined water content is the theoretical value; on the other hand,values of the water contents to be described in the below-discussedpreferred embodiments are measured values. However, as no significantdifference exists between them, it can be assumed that these measuredvalues are the practical values for water content.

In the present invention, by calendaring the gel composition 10 usingthe drum dryer 1 attached to the compressor, the gel composition 10 canbe processed successively. Namely, the absorbent material (I) or theabsorbent material (II) can be manufactured successively.

The resulting sheet-like absorbent materials (I) and (II) have excellentflexibility, and, for example, have the degree of flexibility measuredby the Gurley Stiffness Test of not more than 1,000 mgf, more preferablynot more than 500 mgf, and more preferably not more than 200 mgf, andmost preferably not more than 100 mgf. The method of measuring theflexibility will be explained under preferred embodiments of the presentinvention.

As described, the method (i) of manufacturing the absorbent material (I)of the present invention is a method of reducing the water content whilepressurizing (calendaring) the water-containing gel of the hydrophiliccrosslinked polymer. In the present invention, it is preferable that thewater-containing gel further include a polyhydric alcohol. Furthermore,in the present invention, it is preferable that the water content in thewater-containing gel is in a range of from 30 percent by weight to 90percent by weight. Additionally, the absorbent material (I) is obtainedin particles. However, it is preferable to calendar the water-containinggel to form it into a sheet.

For the manufacturing method (ii) of the absorbent material (II) of thepresent invention, a method of pressurizing the cellular gel of thehydrophiliccrosslinked polymer, preferably, a method of reducing thewater content of the cellular gel under load is adopted. In the presentinvention, it is further preferable that the cellular gel contain thepolyhydric alcohol. Furthermore, the water content in the cellular gelis preferably in a range of from 30 percent by weight to 90 percent byweight. According to the manufacturing method (ii), as in themanufacturing method (i), a particulate absorbent material (II) may beobtained; however, it is preferable to form the cellular gel bycalendaring the cellular gel.

The manufacturing methods (i) and (ii) contain the absorbent resin whichis swollen by absorbing water so as to have anisotropy, therebyproviding absorbent materials (I) and (II) which have excellentproperties particularly in absorbing properties such as absorbing rateand absorbency under load and shape retaining property.

Furthermore, as the water-containing gel and the cellular gel of thehydrophilic cross-linked polymer further include polyhydric alcohol,these gels are formed into a sheet with ease. Therefore, from thesegels, the sheet-like absorbent material can be obtained directly fromthe gel, and the absorbent materials (I) and (II) having excellentflexibility and strength, cushioning characteristic, etc., can beobtained.

Furthermore, in the manufacturing method (i), with a combined use of thesurface cross-linking agent, or by specifying the kind of the main chainof the hydrophilic cross-linked polymer, the water content and thewater-soluble content in the water-containing gel of the hydrophiliccrosslinked polymer, the average particle diameter, the dried averageparticle diameter, etc., of the shape retaining property of theresulting absorbent resin (absorbent resin 30) and the absorbentmaterial (I) containing the absorbent resin can be still improved.

Additionally, in the manufacturing method (ii), with a combined use ofthe surface cross-linking agent, or by specifying the kind of the mainchain of the hydrophilic crosslinked polymer, the water content and thewater-soluble content of the cellular gel of the hydrophilic crosslinkedpolymer, the average particle diameter or the dried average particlediameter of the cellular gel, still improved shape retaining property ofthe absorbent resin (absorbent resin 31) and the absorbent material canbe achieved.

Moreover, according to the manufacturing method (i) or (ii), theabsorbent material (I) or (II) in which the hydrophilic crosslinkedpolymer is immobilized can be manufactured without once forming the gelof the hydrophilic polymer, i.e., the water-containing gel or thecellular gel, into powders. As various processes such as the process ofdrying, pulverizing, classifying, etc., can be eliminated, easy handlingand improved working conditions can be achieved without generating dust.This permits a simplified manufacturing process and an improved yield.As a result, a sheet-like absorbent material can be manufactured at lowcost directly from the water-containing gel or the cellular gel withoutusing a fixing material such as a non-woven fabric. The described methodalso permits an absorbent material having a higher content of thehydrophilic cross-linked polymer, i.e., the absorbent resin to beachieved.

Furthermore, according to the manufacturing method (i) or (ii), theflexibility and the strength can be applied to the resulting absorbentmaterial (I) or (II), and the gel can be calendared into a sheet, theabsorbent material (I) or (II) is wound into a roll, etc., to apply anextension force. As a result, the absorbent material (I) and theabsorbent material (II) can be manufactured successively.

When forming the absorbent materials (I) and (II) into a sheet, pressureand heat are applied to generate a temperature difference between thefirst face and the second face of the sheet.

According to the manufacturing method, by applying pressure and heatsimultaneously to generate a temperature difference between the firstface and the second face of the sheet, a difference occurs in the degreeof distortion of the cross-linked structure of the absorbent resinbetween the first face and the second face of the sheet. Therefore, whenswelling by absorbing water, the retaining of the absorbent resindiffers between the first face and the second face of the sheet, theresulting absorbent material (I) or (II) is swollen while being curledso as to have a curvature. Therefore, when applying the resultingabsorbent material (I) or (II) to the sanitary material such as paperdiaper, sanitary napkin, etc., (absorbent articles) are fit along thebody line and can be prevented from liquid leakage.

The absorbent resin compound (I) resulting from the manufacturing methodcontains the absorbent resin (absorbent resin 30) that is swollen byabsorbing water so as to have anisotropy. Furthermore, the absorbentmaterial (II) resulting from the manufacturing method contains theabsorbent resin (absorbent resin 31) that is swollen by absorbing waterso as to have anisotropy.

Namely, for example, the hydrophilic crosslinked polymer particles asthe absorbent resin (absorbent resins 30 and 31) of the presentinvention are compressed by separating the aqueous solvent from thethree-dimensional net structure of the gel particles when applying thegel particles of the hydrophilic crosslinked polymer. Therefore, thedistortion by compression occurs in the three-dimensional net structureof the absorbent resin (absorbent resins 30 and 31). Therefore, theabsorbent resin (absorbent resins 30 and 31) has a distorted energyinside, and in order to restore the state before being compressed, theabsorbent resin is swollen so as to have anisotropy by quickly absorbingwater in contact water.

As a result, the absorbent resins (absorbent resins 30 and 31) arerestored in the stage before being decompressed without distortion.Namely, the absorbent resin (absorbent resin 30 or 31) retains theoriginal state before being compressed. In the present invention, thecompressed ratio indicates the degree of compression of the absorbentresin (absorbent resins 30 and 31). In order to obtain sufficienteffects of improving absorbency, the compression ratio is in a range offrom 2 to 1,000. As described, the absorbent resin (absorbent resins 30and 31) of the present invention offers various absorbing propertiessuch as excellent absorbing rate and absorbency under applied load,etc., and shape retention as force is exerted to restore the statebefore compression with respect to the distorted cross-linked structure,the absorbency under load, and the shape retaining property.

Additionally, it is preferable that the absorbent resin in accordancewith the present invention has foams inside. As the absorbent resin hasfoams inside, a sufficient space for passing liquid is ensured to allowthe aqueous liquid to move inside the absorbent resin. Therefore, in thecase where foams are included inside the absorbent resin, excellentpermeability to aqueous liquid and diffusion, absorbing rate and thewater retention by capillarity can be achieved.

As described, the absorbent resin (absorbent resins 30, 31) of thepresent invention show excellent various absorbing properties such asabsorbing rate and the absorbency under load, etc., and shape retainingproperty. The absorbent materials (I) and (II) of the present inventionincluding the absorbent resin (absorbent resins 30 and 31) also havevarious absorbing properties such as the absorbing rate and theabsorbency under load, etc., and the shape retaining property.

The shape of the absorbent materials (I) and (II) of the presentinvention self-retains, and thus the amount of the hydrophiliccross-linked polymer, i.e., the absorbent resin per unit area can beincreased as compared to the conventional absorbent material where theabsorbent resin is sandwiched between the absorbent materials such aspulp, non-woven fabric, etc. Therefore, the absorbency and absorbingrate under load can be improved from the conventional arrangement.Namely, the absorbent materials (I) and (II) absorb the aqueous liquidquickly when contacting aqueous liquid such as water, body fluid, drip,etc. The absorbent materials (I) and (II) have the respective watercontents of not more than 80 percent by weight, and if necessary containthe polyhydric alcohol and the subsidiary forming material.Additionally, sheet-like absorbent materials (I) and (II) are obtainedby the described method. They have the absorbing rate of not more than150 seconds, and the absorbency under load of not less than 15 g/g,preferably not less than 20 g/g, more preferably not less than 25 g/gand still more preferably not less than 30 g/g.

When the absorbent materials (I) and (II) contain the polyvalentalcohol, the absorbent materials (I) and (II) show excellentflexibility, strength and cushioning. Additionally, when the absorbentmaterials (I) and (II) have still desirable shape retaining property,improved flexibility and strength can be achieved.

Furthermore, as the absorbent materials (I) and (II) have the absorbentresin (absorbent resins 30 and 31) which is swollen so as to haveanisotropy to retain the original state before being compressed, bymolding such that a difference in distortion between the cross-linkedstructure of the absorbent resin of the heated portion (heating face)and the cross-linked structure of the absorbent resin of the non-heatedportion (heating face) by partially heating the absorbent material (I)or (II), the resulting absorbent material (I) or (II) is swollen whilebeing curled so as to have a curvature even in a flat sheet form.

When adopting the absorbent material for, for example, sanitary material(absorbent article) paper diaper, sanitary napkin, etc., by curling theabsorbent material along the curve of the absorbent material, it can befitted to the body line so as to form a curve to prevent liquid fromleaking.

Additionally, it is preferable that the absorbent materials (I) and (II)are formed into a sheet so as to have a flexibility of not more than1,000 mgf. As a result, the absorbent materials (I) and (II), andsoftness and desirable use can be applied to the absorbent articlehaving the absorbent materials (I) and (II).

The above explanations will be given through the case where theabsorbent materials (I) and (II) are formed into a sheet. However, it ispermitted to form them to have a block shape, a plate shape and a filmshape, etc., and they can be formed into powders by pulverizing, but theforms are not particularly limited.

Additionally, the sheet-like absorbent materials (I) and (II) (sheet) ofthe present invention have a thickness in a range of from 0.01 mm to 5mm, preferably from 0.1 mm to 3 mm, and still more preferably in a rangeof from 0.5 mm to 1 mm. By adjusting the compression ratio, the filmthickness can be adjusted with ease. For example, by adjusting the time,the temperature, the clearance between the compressor and the drumdryer, the compression ratio can be adjusted with ease.

Furthermore, the absorbent materials (I) and (II) may be formed into asheet by once forming it in a powdery form after applying a pressure tothe water-containing gel of the hydrophilic cross-linked polymer or thecellular gel of the hydrophilic cross-linked polymer. In view of theconvenience in handling, working environment, productivity, etc., it ispreferable that the water-containing gel and the cellular gel containpolyhydric alcohol, and that these gels are formed directly into sheetswithout once forming them into powders.

Although explanations have been given through the case where theabsorbent material (I) and the absorbent material (II) having such astructure that a mixture of the water-containing gel or the cellular geland polyhydric alcohol are mixed with a fiber as a subsidiary formingmaterial, the structure of the absorbent material (I) and the absorbentmaterial (II) is not particularly limited.

Other than the described mixed structure of the mixture and the fiber,the absorbent material (I) and the absorbent material (II) may have thestructure wherein the mixture is held by a plurality of fibers formedinto a sheet such as non-woven fabrics, woven fabrics; paper, etc.; thearrangement wherein after mixing the mixture and the fibers to be formedinto sheets, the resulting fibers formed into a sheet are held by aplurality of the sheets; a structure wherein after mixing the mixtureand the fibers to be formed into sheets, the sheet is sandwiched by aplurality of fibers formed into a sheet, etc.

The absorbent article of the present invention contains at least oneabsorbent material selected from the group consisting of the absorbentmaterial (I) and the absorbent material (II) having the describedstructure. Namely, the absorbent materials (I) and (II) themselves canbe formed into the absorbent article, or the absorbent material (I) andthe absorbent material (II) can be combined with other material. Thestructure of the absorbent article is not particularly limited. In thecase of applying the absorbent article, for example, to paper diapers(disposal diapers), sanitary napkins, incontinence pads, it ispreferable to adopt a) a structure wherein the sheet like absorbentmaterial (I) or the absorbent material (II) having the describedarrangement is sandwiched by a liquid permeable sheet and a liquidimpermeable sheet or b) a structure wherein the sheet like absorbentmaterial (I) or the absorbent material (II) having the describedarrangement is sandwiched between two liquid permeable sheets. Theabsorbent material (I) and the absorbent material (II) as the absorbentlayer have excellent absorbing properties. Therefore, when adopting theabsorbent article to paper diapers, the leakage of urine from the paperdiapers can be prevented, and the user of the paper diaper can feel dry.

The sheet which is permeable to liquid (hereinafter referred to asliquid permeable sheet) is made of a material that is permeable toaqueous liquid. Examples of such liquid permeable sheet include but arenot limited to: webs or mats, for example, regenerated cellulose fibers,non-woven fabrics, etc., woven fabrics made of a synthetic fiber such asrayon, etc., cotton card web, and the like; cotton pulp, paper; poroussynthetic resin films made of polyethylene, polypropylene, polyesters,polyamide, and the like. The liquid permeable sheet may be formed into abag having a size sufficient for storing the absorbent material.

The sheet that is not permeable to liquid (hereinafter referred to asliquid impermeable sheet) is made of a material which is not permeableto liquid. Examples of a material of such liquid impermeable sheet maybe but are not limited to: synthetic resin film such as polyamide,polyethylene, polypropylene, polystyrene, poly vinyl chloride, etc.; afilm made of a composite material of the synthetic resin and non-wovenfabrics; a film made of a composite material of the synthetic resin andthe woven fabrics, and the like. The liquid impermeable sheet ispermeable to steam.

The method of manufacturing the absorbent article of the presentinvention is not particularly limited. The absorbent article ismanufactured, for example, by laminating at least one absorbent materialselected from the group consisting of the absorbent material (I) and theabsorbent material (II), and the liquid impermeable sheet. Additionally,the sheets thus laminated may be subjected to a further process such asbonding the circumferential portion of the sheets to be immobilized, orpartially bonding the sheets to be immobilized, or forming a slit or, aprocess such as an embossing process, etc. The absorbent product may beformed by laminating the liquid permeable sheet or the liquidimpermeable sheet onto one surface of the sheet like absorbent material(absorbent material (I) or the absorbent material (II)). Furthermore,the absorbent material, i.e., the absorbent article may be formed byfirst placing (applying) the gel or the gel composition onto the liquidpermeable sheet or the liquid impermeable sheet, and then applyingpressure (calendaring). Furthermore, the absorbent article may be formedby first cutting the sheet like the absorbent material (I) and theabsorbent material (II) into a strip, and then mixing it with cellulosefibers, etc.

In reference to FIG. 4, paper diapers as one application of theabsorbent article of the present invention will be explained. Here, theapplication of the absorbent article of the present invention is notlimited to the paper diapers.

As shown in FIG. 4, the paper diapers of the present invention arecomposed of a back sheet 21 (liquid impermeable sheet), an absorbentmaterial 22 (the sheet-like absorbent material (I) or the sheet-likeabsorbent material (II)) and a top sheet 23 as a liquid permeable sheet,etc. The back sheet 21 and the top sheet 23 are formed into apredetermined shape. Then, the back sheet 21, the absorbent material 22and the top sheet 23 are laminated in this order by a both-sided tape,etc. Then, a leg gather 24 and a waist gather 25 are formed at apredetermined position of the sheets thus laminated, and tape fasteners26 are mounted at a predetermined position of the sheets, therebypreparing paper diapers as one application of the absorbent article.

The resulting paper diapers prepared from the absorbent material 22 showexcellent absorbing properties such as absorbing rate, absorbency underapplied pressure, and shape retaining property, and also show excellentflexibility and strength. Thus, the paper diapers permit the urine to bequickly absorbed, thereby providing desirable paper diapers which show adesirable use without leakage of urine, etc.

Moreover, as the absorbent resin 22 is swollen while being curled byabsorbing water, the paper diaper can be fitted along the body line andoffers an improved effect of preventing a leakage of liquid.

As described, as the absorbent product includes the absorbent material(at least one absorbent material selected from the group selected fromthe group consisting of the absorbent material (I) and the absorbentmaterial (II)), the absorbent product which is excellent in variousabsorbing properties such as absorbing rate, absorbency under load,etc., and is soft and comfortable to use can be provided. As a result,the absorbent article which is comfortable to the skin of the user canbe achieved.

Conventionally, the ratio of the hydrophilic cross-linked polymer basedon the total weight of the absorbent product is relatively low, i.e.,less than 40 percent by weight. In contrast, the described arrangementof the present invention has a high ratio of the hydrophiliccross-linked polymer based on the total weight of the absorbent product,i.e., generally in a range of from around 40 percent by weight to around80 percent by weight, preferably from around 50 percent by weight toaround 80 percent by weight, and still more preferably in a range offrom around 60 percent by weight to around 80 percent by weight.

Accordingly, the present invention offers a thinner and compact sizeabsorbent product while maintaining the absorbing properties in the samelevel as those of the conventional absorbent articles. Furthermore, asthe absorbent materials (I) and (II) are formed by applying pressure tothe water-containing gel and cellular gel, in processes includingmanufacturing, wrapping and transporting processes of the absorbentmaterial (I) or the absorbent material (II) or the absorbent product,the hydrophilic cross-linked polymer does not move in the absorbentmaterials (I) and (II), and the absorbent materials (I) and (II) can beprevented from being dropped off. Furthermore, for example, when formingthe absorbent materials (I) and (II) into sheets, different from theconventional absorbent material prepared by once forming the absorbentresin into a powdery form and then forming the resulting powderyabsorbent resin into a sheet, the absorbent materials (I) and (II) ofthe present invention are kept in a sheet form even after being swollenby absorbing water. Thus, even after being swollen by absorbing water,the gel can be prevented from being dropped from the absorbent material(I) or the absorbent material (II).

The absorbent material (I) or the absorbent material (II) may optionallyinclude a deodorant, perfume, various chemical agents, various inorganicagents, water-soluble polymers, plant growth accelerators,anti-bacterial agents, mildewproofing agents, foaming agents, pigments,dyes, carbon blacks, activated carbons, hydrophilic short fibers,fertilizers, oxidizing agents, reducing agents, water, salts, etc. toprovide additional functions to the absorbent material (I) or (II) or tothe absorbent article.

The absorbent material (I), the absorbent material (II) and theabsorbent article of the present invention are suitable for use insanitary materials such as paper diapers (disposable diapers), sanitarynapkins, and tampons, so-called incontinence pads, etc., moisturecondensation absorbent sheets, agricultural water retaining materials,waterproofing agents for civil engineering works, medical materials,such as medical sheets, bed pads, etc., materials for keepingfood-stuffs fresh, and materials for preventing foodstuffs fromdripping, towels, bandages, etc., for the purpose of absorbing aqueoussolution and moisture. The applications of the absorbent material (I)and the absorbent material (II) and the absorbent material are notlimited to the above examples.

BRIEF EXPLANATIONS OF THE DRAWINGS

FIG. 1 which shows one example of the manufacturing method of anabsorbent material of the present invention is a cross-sectional viewschematically showing a drum dryer provided with a calendaring device;

FIG. 2(a) is an explanatory view showing a compressed state of thenon-cellular absorbent resin contained in the absorbent material of thepresent invention;

FIG. 2(b) is an explanatory view showing the state where the absorbentresin of FIG. 2(a) is swollen by absorbing water;

FIG. 3(a) is an explanatory view showing a compressed absorbent resinhaving foams inside in another absorbent material of the presentinvention;

FIG. 3(b) is an explanatory view showing a state where the absorbentresin shown in FIG. 3(a) is swollen by absorbing water; and

FIG. 4 is a perspective view schematically showing a partial cut offsurface of a paper diaper as an example of the absorbent articleadopting the absorbent material of the present invention.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

The present invention will be described in detail by way of examples andcomparative examples. However, the present invention is not limited tothe disclosure below. The performances of absorbent materials weremeasured in the following manner.

(a) Water Content

First, a sheet-like absorbent material was cut into a size of 10 cmsquare, and the weight W₀ (g) of the absorbent material (hereinafterreferred to as a cut sheet) was measured. The cut sheet was placed in anoven maintained at 180° C. (Tokyo Rika Kiki Co., Ltd.: type NDO-450) andwas left for 3 hours. Then, the cut sheet was taken out, and was placedin a desiccator in which silicagel was placed, and the cut sheet wascooled off for 5 minutes. Then, the weight W₁ (g) of the sheet wasmeasured. By substituting the weight W₀ (g) and the weight W₁ (g) intothe following formula, the water content (percent by weight) wasdetermined.

    ______________________________________                                        Water content (wt %)                                                              = [(weight W.sub.0  (g) - weight W.sub.1  (g))/ Weight W.sub.0  (g)]          ×                                                                         100.                                                                    ______________________________________                                    

(b) Absorbency

Here, 0.2 g of the absorbent material was uniformly placed into a teabag (60 mm×60 mm) made of non-woven fabric, and the opening of the teabag was heat sealed. Then, the tea bag was immersed into a 0.9 percentby weight sodium chloride solution (physiological saline solution) for30 minutes. Then, the tea bag was taken out, and was subjected tohydro-extraction for 3 minutes at 250 G using a centrifugal separator,and the weight W₂ (g) of the tea bag was measured. Further, the sameprocesses were carried out with an empty bag, and the weight W₃ (g) ofthe empty tea bag was measured. The absorbency (g/g) of the sampledabsorbent material was determined by substituting the weight W₂ and theweight W₃ into the following formula:

    ______________________________________                                        Absorbency (g/g)                                                                  =(Weight W.sub.2  (g) - Weight W.sub.3  (g))/ Weight (g) of the                Absorbent Material.                                                      ______________________________________                                    

(c) Absorbency Under Load

First, in a glass petri dish having an inner diameter of 160 mm and aheight of 20 mm, a glass filter plate (G#1) having a diameter of 120 mmwas placed. Then, a 0.4 percent by weight aqueous saline solution waspoured into the petri dish. The aqueous saline solution was added in anamount such that the surface of the aqueous saline solution was insubstantially the same level as the filter plate. Next, a filter paper(TOYO FILTER PAPER No. 2 available from Toyo Filter Co., Ltd.) wasplaced on the filter plate.

Then, for the sheet-like absorbent material in a sheet form, it was cutinto a size of 3.1 cm square, and the weight W₄ (g) of the absorbentmaterial (hereinafter referred to as a cut sheet) was measured. Then, ametal gauze made of stainless steel having a 400-mesh was fixed on thebottom portion of an acrylic resin cylinder having an inner diameter of55 mm and a height of 60 mm, thereby preparing a support cylinder. Thecut sheet was placed in the support cylinder, i.e., on the metal gauze,and a cylindrical brass plunger was placed as a weight, therebypreparing a measuring cylinder. The weight of the plunger was adjustedso as to be capable of uniformly applying a load of 50 g/cm². Then, thetotal weight of the cut sheet, the support cylinder and the plunger,i.e., the weight W₅ (g) of the measuring cylinder was measured.

Then, the measuring cylinder was placed on the filter paper. The salinesolution was absorbed by the cut sheet in the measuring cylinder for 30minutes after the measuring cylinder was placed on the filter paper.Namely, a 0.4 percent by weight saline solution was absorbed by the cutsheet under load. While the saline solution was being absorbed by thecut sheet, a 0.4 percent by weight saline solution was added to thepetri dish until the surface of the saline solution became substantiallythe same level as the upper surface of the filter plate. After an elapseof time of 30 minutes, the weight W₆ of the cylinder was measured. Then,by substituting the resulting weights W₄, W₅ and W₆ into the followingformula, the absorbency under load (g/g) was determined.

    Absorbency under Load (g/g)=(Weight W.sub.6 (g)-Weight W.sub.5 (g))/Weight W.sub.4 (g)).

On the other hand, for the powdery absorbent material, in place of thesheet of a size of 3.1 cm square, 0.9 g of the absorbent material wasuniformly dispersed in the acrylic resin cylinder, and the absorbencyunder load of 50 g/cm² was measured. Then, the absorbency under load(g/g) was determined according to the above formula.

(d) Absorbing Rate

For a sheet-like absorbent material, first, it was cut into a size of2.54 cm square (1 inch²), and the weight of the absorbent material(hereinafter referred to as a cut sheet) was measured. On the otherhand, into a polypropylene container having an inner diameter of 55 mmand a height of 15 mm, a physiological saline solution was poured into aweight of 10 times based on the weight of the cut sheet, and the cutsheet was placed in the physiologic saline solution. Then, the timerequired for the saline solution to be completely absorbed by the cutsheet after the cut sheet was placed in the physiologic saline solutionwas measured, and the resulting time was determined to be the absorbingrate (seconds). In this measurement, it was checked to see if thephysiological saline solution had been completely absorbed by the cutsheet by the existence of the remaining liquid of the physiologicalsaline solution by tilting the polypropylene container at 45°. Namely,the point from which the remaining liquid no longer existed wasdetermined to be the point where all the physiological saline solutionwas absorbed by the cut sheet.

In the case of the powdery absorbent material, in place of the sheet ofa size of 2.54 cm square, the absorbing rate was measured using 1 g ofthe absorbent material in the same manner.

(e) Flexibility

The flexibility was measured with respect to only the sheet-likeabsorbent material according to the Gurley Stiffness Test defined in JISL 1096. The flexibility was determined such that the smaller value(flexibility) measured by the method indicated the higher flexibility.

EXAMPLE 1

A jacketed stainless steel kneader having an inner volume of 10 litersand provided with two sigma shaped vanes having a rotation diameter of120 mm was prepared as a reactor. The kneader has a lid for sealing theinside of the system, and a lid for applying a pressure of 66 kg tocontents placed therein. In the reactor, 5,000 g of 38 percent by weightaqueous solution of acrylic acid (monomer component) and sodium acrylate(monomer component) (75 mole percent of the total monomer components inthe aqueous solution is neutralized) and 2.85 g of trimethylolpropanetriacrylate as a cross-linking agent (hereinafter referred to as across-linking agent (A)) were placed and nitrogen gas was blown in todisplace the air entrapped in the reaction system. The amount of use ofthe cross-linking agent (A) with respect to the monomer component was0.045 mole percent.

The kneader was heated by passing hot water at 30° C. through thejacket, while stirring the contents of the kneader, and sodiumpersulfate and L-ascorbic acid were added as polymerization initiatorsto the contents of the kneader. As a result, the polymerization startedafter an elapse of time of around 1 minute. Based on the monomercomponent, the respective amounts of use of the sodium persulfate andL-ascorbic acid were 0.12 mole percent and 0.005 mole percentrespectively.

Then, after carrying out a polymerization at 30° C. for 60 minutes, thepressurizing lid was placed on the resulting bulk-like water-containinggel (contents), and the gel was stirred for 20 minutes and was dividedinto fine pieces. As a result, finely divided water-containing gelhaving a particle diameter in a range of from 0.2 mm to 0.8 mm wasobtained. The solid portion of the water-containing gel was 38 percentby weight.

Then, 800 g of finely divided water-containing gel was placed in anotherkneader having the same arrangement as the described kneader. Then, amixed solution of 33.8 g of glycerol as a polyvalent alcohol and 0.31 gof ethylene glycol diglycidyl ether as surface cross-linking agent(Denacol EX-810 available from Nagase Chemical Industry Co., Ltd.,hereinafter referred as a surface cross-linking agent (B)) was prepared.

The kneader was heated by passing hot water at 70° C. through thejacket, while stirring the contents of the kneader. Then, with stirring,the mixed solution was added to the water-containing gel, and wasstirred until both are uniformly mixed. Based on 100 parts by weight ofthe solid portion of the water-containing gel, the amounts of use ofglycerol and the surface cross-linking agent (B) were 11.1 parts byweight and 0.1 parts by weight respectively.

Next, to the resulting mixture, 10.4 g of polyester fibers (auxiliarymolding compound) having a length of from 2 mm to 3 mm made ofpolyethylene terephtalate was added to the resulting mixture little bylittle and was kneaded until the agglomeration of the fibersdisappeared. The amount of the polyester fiber with respect to 100 partsby weight of the solid portion of the water-containing gel was 3.4 partsby weight, thereby obtaining a water-containing gel composition.

Thereafter, the water-containing gel composition was calendared by anopen-type single drum dryer (available from Katsuragi Co., Ltd., TYPE:NRXM 750-N35C) and the compressor provided therewith, and a contactsurface between the water-containing gel composition (1) and the drumdryer was heated to 150° C. Thereafter, the resulting sheet was scrapedoff by the scraper from the surface of the drum dryer. As a result, asheet-like absorbent material having a thickness of 0.8 mm was obtained.The main conditions for manufacturing the water-containing gel aresummarized in Table 1.

The resulting absorbent material had a weighing of 490 g/m², a watercontent of 7.5 percent by weight, an absorbency of 30.2 g/g, anabsorbency under load of 25.2 g/g, and an absorbing rate of 68 seconds.These results of measurements were summarized in Table 2.

COMPARATIVE EXAMPLE 1

The water-containing gel composition resulting from Example 1 was heatedto 150° C. by means of only the drum dryer without using the compressor,and was formed into a sheet without calendaring. Thereafter, theresulting sheet was scraped off by the scraper from the surface of thedryer drum. As a result, a sheet-like absorbent material was obtained.The results of measurements of the resulting absorbent material weresummarized in Table 2.

COMPARATIVE EXAMPLE 2

The water-containing gel composition resulting from Example 1 wascalendared without heating the dryer drum. As a result, thewater-containing gel composition was formed into a sheet withoutreducing the water content of the water-containing gel. Thereafter, theresulting sheet was scraped off using the scraper from the surface ofthe drum dryer (temperature of 25° C.). As a result, a sheet-likeabsorbent material was obtained. The results of measurements of theresulting absorbent material are summarized in Table 2.

COMPARATIVE EXAMPLE 3

The sheet-like absorbent material resulting from comparative example 2was dried in a hot air circulating type dryer maintained at 105° C.Namely, after the water-containing gel was calendared, the water contentwas reduced. The results of measurements of the absorbent material aresummarized in Table 2. The resulting absorbent material showed poorshape retaining property, and the flexibility of the sample could not bemeasured.

EXAMPLE 2

Except that 76 g of glycerol was used in place of the mixed solution ofExample 1, the reaction and operations of example 1 were repeated,thereby obtaining a sheet-like absorbent material. In this example, 25parts by weight of glycerol were used based on 100 parts by weight ofthe solid portion of the water-containing gel. The main conditions formanufacturing the water-containing gel composition are summarized inTable 1. Additionally, the results of measurements of the resultingabsorbent material are summarized in Table 2.

EXAMPLE 3

Except that a mixture of 76 g of glycerol and 0.31 g of the surfacecross-linking agent (B) was used in place of the mixed solution ofExample 1, the reaction and operations of Example 1 were repeated,thereby obtaining a sheet-like absorbent material. The main conditionsof manufacturing the water-containing gel composition are summarized inTable 1. Additionally, the results of measurements of the resultingabsorbent material are summarized in Table 2.

EXAMPLE 4

Except that a mixture of 76 g of glycerol and 1.53 g of the surfacecross-linking agent (B) was used in place of the mixed solution ofExample 1, the reaction and operations of Example 1 were repeated,thereby obtaining a sheet-like absorbent material. In this example, 0.5parts by weight of the surface cross-linking agent (B) were used basedon 100 parts by weight of the solid portion of the water-containing gel.The main conditions of manufacturing the water-containing gelcomposition are summarized in Table 1. Additionally, the results ofmeasurements of the resulting absorbent material are summarized in Table2.

EXAMPLE 5

Except that a mixture of 76 g of glycerol and 1.53 g of the surfacecross-linking agent (B) was used in place of the mixed solution ofExample 1, and that the amount of use of the polyester fiber was alteredfrom 10.4 g to 16.0 g, the reaction and operations of Example 1 wererepeated, thereby obtaining a sheet-like absorbent material. In thisexample, 5.3 parts by weight of the surface polyester fibers were usedbased on 100 parts by weight of the solid portion of thewater-containing gel. The main manufacturing conditions of thewater-containing gel composition are summarized in Table 1.Additionally, the results of measurements of the resulting absorbentmaterial are summarized in Table 2.

EXAMPLE 6

Except that 304 g of glycerol were used in place of the mixed solutionof Example 1, the reaction and operations of Example 1 were repeated,thereby obtaining a sheet-like absorbent material. Based on 100 parts byweight of the solid portion of the water-containing gel, the amount ofuse of glycerol was 100 parts by weight. The main conditions ofmanufacturing the water-containing gel composition are summarized inTable 1. Additionally, the results of measurements of the resultingabsorbent material are summarized in Table 2.

EXAMPLE 7

Except that an amount of use of the cross-linking agent (A) of Example 1was altered from 2.85 g to 1.27 g, and a mixed solution of 76 g ofglycerol and 1.53 g of the surface cross-linking agent (B) was used inplace of the mixed solution of Example 1, the reaction and theoperations of Example 1 were repeated, thereby obtaining a sheet-likeabsorbent material. In this example, 0.02 mole percent of thecross-linking agent (A) were used with respect to the monomer component.The main conditions of manufacturing the water-containing gelcomposition are summarized in Table 1. Additionally, the results ofmeasurements of the resulting absorbent material are summarized in Table2.

EXAMPLE 8

Except that 5,000 g of 30 percent by weight aqueous solution of acrylicacid (monomer component) and sodium acrylate (monomer component) (75mole percent of the total monomer components in the aqueous solution isneutralized) were used in place of an aqueous 38 percent by weight ofsodium acrylate, and 7.18 g of polyethylene glycol diacrylate (ethyleneglycol had an average additional number of moles of 8; hereinafterreferred to as a cross-linking agent (C)) as a cross-linking agent, thereaction and operations of Example 7 were repeated, thereby obtaining asheet-like absorbent material. In this example, 0.07 mole percent of thecross-linking agent (C) was used based on the monomer component.Additionally, the solid portion of the water-containing gel was 30percent by weight. The main conditions of manufacturing thewater-containing gel composition are summarized in Table 1.Additionally, the results of measurements of the resulting absorbentmaterial are summarized in Table 2.

EXAMPLE 9

Except that a mixture of 76 g of glycerol and 0.31 g of the surfacecross-linking agent (B) was used in place of the mixed solution ofExample 1, and that the polyester fibers were not used (not added), thereaction and the operations of Example 1 were repeated, therebyobtaining a sheet-like absorbent material. The main conditions ofmanufacturing the water-containing gel composition are summarized inTable 1. Additionally, the results of measurements of the resultingabsorbent material are summarized in Table 2.

EXAMPLE 10

Except that a mixture of 76 g of glycerol and 1.53 g of the surfacecross-linking agent (B) was used in place of the mixed solutionresulting from Example 1, and that 76 g of pulp fibers (auxiliarymolding compound) having a fiber length of from around 10 mm to 20 mmwas used in place of the polyester fibers, the reaction and operationsof Example 1 were repeated, thereby obtaining a sheet-like absorbentmaterial. In this example, 25 parts by weight of the pulp fibers wereused based on 100 parts by weight of the solid portion of thewater-containing gel. The main conditions of manufacturing thewater-containing gel composition are summarized in Table 1.Additionally, the results of measurements of the resulting absorbentmaterial are summarized in Table 2.

EXAMPLE 11

Except that polyester fibers having a fiber length of from around 20 mmto 30 mm were used, the reaction and the operations of Example 1 wererepeated, thereby obtaining a sheet-like absorbent material. Theabsorbent material resulting from this example was superior to theabsorbent material resulting from Example 1 in its shape retainingproperty in both dried and swollen states and the absorbing rate. Themain conditions of manufacturing the water-containing gel compositionare summarized in Table 1. Additionally, the results of measurements ofthe resulting absorbent material are summarized in Table 2.

                  TABLE 1                                                         ______________________________________                                                             Polyester                                                  Fibers                                                                      A        B          C      D     E    F                                       ______________________________________                                        1      38    0.045      11.1 0.1   3.4  2-3                                     2 38 0.045 25 0 3.4 2-3                                                       3 38 0.045 25 0.5 3.4 2-3                                                     4 38 0.045 25 0.5 3.4 2-3                                                     5 38 0.045 25 0.5 5.3 2-3                                                     6 38 0.045 100 0 3.4 2-3                                                      7 38 0.02 25 0.5 3.4 2-3                                                        Cross-                                                                      8 30 Linking 25 0.5 3.4 2-3                                                     Agent (C)                                                                     0.07                                                                        9 38 0.045 25 0.1 0   2-3                                                          Pulp                                                                     10 38 0.045 25 0.5 Fiber 10-20                                                     25                                                                       11 38 0.045 11.1 0.1 3.4 20-30                                              ______________________________________                                         A: Solid Portion (% by weight)                                                B: CrossLinking Agent (A) (mole %)                                            C: Glycerol (Parts by Weight)                                                 D: Surface CrossLinking Agent (B) (parts by weight)                           E: (parts by weight)                                                          F: (mm)                                                                  

                  TABLE 2                                                         ______________________________________                                                 A       B       C     D     E     F                                  ______________________________________                                        EXAMPLE 1                                                                              490     7.5     30.2  25.2  68    750                                  COMP. 510 8.2 29.8 24.4 92 930                                                EXAMPLE 1                                                                     COMP. 990 54.1 15.5 9.8 270 --                                                EXAMPLE 2                                                                     COMP. 520 9.9 28.8 23.4 97 1020                                               EXAMPLE 3                                                                     EXAMPLE 2 753 11.2 32.1 15.3 114 520                                          EXAMPLE 3 474 16.2 24.2 25.1 71 260                                           EXAMPLE 4 509 17.4 20.5 27.5 47 160                                           EXAMPLE 5 273 12.9 22.2 27.5 23 490                                           EXAMPLE 6 1108 17.8 21.6 17.8 83 310                                          EXAMPLE 7 470 14.2 21.5 27.7 31 300                                           EXAMPLE 8 510 20.0 18.1 26.7 36 150                                           EXAMPLE 9 650 12.5 25.7 27.0 92 500                                           EXAMPLE 10 1056 6.6 18.2 22.9 55 1000                                         EXAMPLE 11 490 7.4 30.5 25.1 60 840                                         ______________________________________                                         A: Weighing (g/m.sup.2)                                                       B: Water Content (% by Weight)                                                C: Absorbency (g/g)                                                           D: Absorbency under Load (g/g)                                                E: Absorbing Rate (seconds)                                                   F: Flexibility (mgf)                                                     

As can be seen from the results shown in Table 2, by reducing the watercontent from the water-containing gel under an applied load, even whenadopting the water-containing gel composition, compared with the case ofapplying only either one of heat and pressure, or the case of applyingheat and pressure separately, the absorbent material which shows stillimproved absorbing properties including absorbency under load and theabsorbing rate can be obtained. Furthermore, according to the presentembodiment, a sheet-like absorbent material which is excellent in itsabsorbency under load, absorbing rate and flexibility can be obtained.

Additionally, the compression ratio was measured with respect to therespective sheet-like absorbent materials of Examples 1 through 11. Thecompression ratio of the absorbent materials of Examples 1 through 11falls in a range of from 10 to 100, and the absorbent materials showexcellent properties in their absorbing rates, etc., compared with theabsorbent materials of comparative examples 1 through 3. Not only forthe primary order particles of the water-containing gel, but also forthe sheet-like absorbent materials, the compression ratio can bedetermined in the same manner as the aforementioned method.Additionally, the absorbent resin in each absorbent material recovers inthe original state before being compressed by swelling (calendaring),thereby showing an excellent absorbency retaining property.

EXAMPLE 12

The reaction and operations of Example 1 were repeated to obtain awater-containing gel. Then, without mixing glycerol (polyvalentalcohol), the surface cross-linking agent (B), and the polyester fibers(subsidiary molding compounds) with the water-containing gel, thewater-containing gel was calendared and heated in the same manner asExample 1. Namely, the calendaring and heating processes were carriedout with respect to only the water-containing gel as thewater-containing gel composition.

Thereafter, 200 g of the resulting absorbent material were placed in themixer, and were pulverized into a powdery form for 15 seconds. Next, thepowdery absorbent material was classified by the JIS standard sievehaving an opening of 500 μm to remove coarse granules, thereby obtaininga powdery absorbent material in accordance with the present invention.

The absorbent material resulting from calendaring and heating thewater-containing gel was very fragile, and a sheet-like absorbentmaterial could not be obtained directly from the water-containing gel.Namely, when calendaring and heating the water-containing gel in apresence of a polyhydric alcohol, a sheet-like absorbent material couldnot be obtained. However, the powdery absorbent material resulting fromdrying and subsequently classifying the absorbent material showedexcellent absorbing properties such as absorbency of 42.5 g/g, theabsorbency under load of 11.3 g/g, and absorbing rate of 25 seconds. Theresults of measurement of the absorbent material are described in Table3.

COMPARATIVE EXAMPLE 4

The water-containing gel of Example 12 was dried in hot air for 1 hour,and was classified by the JIS standard sieve having an opening of 500 μmafter pulverizing 200 g of the resulting dried material by a desk toppulverizer, thereby obtaining a powdery absorbent material by removingcoarse granules. Then, the results of measurement of the absorbentmaterial are shown in Table 3.

COMPARATIVE EXAMPLE 5

A mixture of the resulting powdery absorbent material from ComparativeExample 4 and the polyhydric alcohol was pressurized under an appliedheat according to the method of U.S. Pat. No. 4,066,583.

Namely, first, in a predetermined container, 8 g of the powderyabsorbent material resulting from Comparative Example 5 and 2 g ofglycerol (polyvalent alcohol) were placed, and they were quickly mixeduniformly, thereby obtaining a powdery mixture. After uniformly placingthe mixture in a size of 10 cm square, the mixture was placed in theisothermic room having constant humidity adjusted to have a temperatureof 25° C. and a relative humidity of 90 percent, and was left for 10minutes to improve properties of the mixture. After the humidity wasapplied to the mixture, the powdery absorbent material was formed into asheet in which powdery absorbent materials are agglomerated, and had ashape retaining propriety which allows it to be lifted up slowly.

Next, using a heat application type compressor, pressure was applied tothe sheet-like mixture for 5 minutes at a temperature of 150° C. andunder load of 350 gf/cm². As a result, a transparent sheet-like moldinghaving a thickness of 1 mm was obtained. Next, by adding water to theresulting sheet-like molding so as to have a water content of 17.0percent by weight, the sheet-like absorbent material was obtained. Theresults of measurements of the absorbent material were summarized inTable 3.

EXAMPLE 13

The reaction and operations of Example 1 were repeated to obtain awater-containing gel. Next, after mixing only 0.31 g of the surfacecross-linking agent (B) to the water-containing gel, calendaring andheating processes were applied in the same manner as Example 1. Namely,using a mixture of the water-containing gel and the surfacecross-linking agent (B) as a water-containing gel composition,calendaring and heating processes were carried out. Thereafter, 200 g ofthe resulting absorbent material was pulverized and classified in thesame manner as Example 12, thereby obtaining powdery absorbent material.

The absorbent material obtained in this Example by calendaring andheating the water-containing composition was also very fragile, and asheet-like absorbent material could not be obtained. Namely, as thewater-containing gel was subjected to the calendaring and heatingprocesses in an absence of polyhydric alcohol, the sheet-like absorbentmaterial could not be obtained. However, by carrying out the processesof calendaring and heating the water-containing gel compositionsimultaneously, an absorbent material which shows excellent absorbingproperties such as the absorbing rate, the absorbency under load,especially the absorbing rate could be obtained. The results ofmeasurements of the absorbent material are also shown in Table 3.

COMPARATIVE EXAMPLE 6

Except that instead of calendaring and heating the water-containing gelcomposition, the water-containing gel was dried for 1 hour in a hot airat 160° C., and after pulverizing 200 g of the resulting dried materialby the desk top pulverizer, they were classified by the JIS standardsieve with an opening of 500 μm to remove coarse granules, the reactionand operations of Example 13 were repeated, thereby obtaining a powderyabsorbent material. The results of measurements of the absorbentmaterial are also described in Table 3.

COMPARATIVE EXAMPLE 7

2 g of glycerol was mixed with 8 g of powdery absorbent materialresulting from Comparative Example 6, and in the same manner asComparative Example 5, a sheet-like absorbent material having athickness of 1 mm was obtained. Then, to the sheet-like molding article,water was added so as to have a water content of 19.2 percent by weight,thereby obtaining a sheet-like absorbent material.

The results of measurements of the absorbent material are summarized inTable 3.

EXAMPLE 14

In a 20 liters reaction container equipped with an agitator, a refluxcondenser, a thermometer, a nitrogen gas inlet tube and a droppingfunnel, 10 litters of cyclohexane and 40 grams of sucrose fatty acidester (trade name: DK-ESTER F-50, available from Dai-Ichi Kogyo SeiyakuLtd.) as a surface active agent having 6 HLBs (hydrophile-lipophilebalance) were placed, and the content in the reaction container wasstirred. After the sucrose fatty acid ester was dissolved in thecyclohexane, nitrogen gas was introduced in the reaction container toreplace the air with nitrogen gas.

On the other hand, in 3,030 g of 35 percent by weight aqueous solutionof acrylic acid (monomer component) and sodium acrylate (monomercomponent) (75 mole percent of the total monomer components in theaqueous solution is neutralized), 0.16 g of N,N'-methylenebisacrylamides as a cross-linking agent and 5.3 g ofhydroxyethylcellulose (as a thickening agent (trade name: EP-850,available from Daicel Chemical Industries Ltd.) were dissolved to obtaina monomer solution. Then, after removing the oxygen remaining in thesolvent by introducing nitrogen gas into the monomer aqueous solution,1.6 g of potassium persulfate were added as a polymerization initiatorto be dissolved therein.

Thereafter, the monomer solution having the polymerization initiatordissolved therein was added to the aqueous solution in the reactioncontainer as a reaction solution. Then, the reaction solution wassubjected to a reverse phase suspension polymerization while stirringthe reaction solution for two hours at 60° C., thereby obtaining asphere gel-like polymer. Then, the resulting gel-like polymer wasdehydrated by forming an azeotrope and the gel-like polymer was reactedin the reaction container, thereby obtaining dehydrated polymer havingan average particle diameter of 480 μm. Thereafter, the resultingdehydrated polymer was classified by a JIS standard sieve to have thedehydrated polymer having a uniform particle diameter in a range of from500 to 600 μm.

Thereafter, to 100 parts by weight of the dehydrated polymer, 0.05 partsof ethylene glycol diglycidyl ether (surface cross-linking agent (B)) asa cross-linking agent, and an aqueous solution of a crosslinking agentcomposed of 3 parts by weight of water and 2 parts by weight ofisopropanol were added, thereby obtaining a secondary cross-linkedpolymer by carrying out a surface cross-linkage at 200° C. The watercontent of the gel-like polymer was reduced without applying a pressure.As a result, the secondary cross-linked polymer is a complete sphereshape having an average particle diameter of 480 μm.

Thereafter, the secondary cross-linked polymer was expanded by addingwater, thereby obtaining a water-containing gel of the secondarycross-linked polymer having a 35 percent by weight solid portion. Thereaction and operations of Example 1 were repeated with respect to thewater-containing gel of the secondary cross-linked polymer withapplications of heat and pressure, thereby obtaining the absorbentmaterial in a powdery form.

After undergoing calendaring and heating processes, 100 disk-shapedsecondary cross-linked polymer particles having an average thickness of0.07 mm and an average diameter of 2.60 mm were obtained. Therefore, thecompression ratio X in the compressing direction (thickness direction)of the secondary cross-linked polymer particles after undergoingcalendaring and heating processes was 0.07/0.48, the expansion ratio Yin the milling direction was 2.60/0.48, and the compression ratio Y/Xdefined in the present invention was 37. Namely, the secondarycross-linked polymer particles in the absorbent material were compressedby 37 times compared with the state before undergoing calendaring andheating processes. Therefore, the absorbent material composed of thesecondary cross-linked polymer particles had high absorbing rate (48seconds) by a distortion energy in the secondary cross-linked polymerparticles, and the absorbent resin was expanded when absorbing water,and was restored into a shape (spherical shape). The results of themeasurements of the absorbent material are summarized in table 3.

COMPARATIVE EXAMPLE 8

Reaction and operations of Example 14 were repeated, and a secondarycross-linked polymer was obtained. The respective performances of theresulting secondary cross-linked polymer were measured directly as acomparative absorbent material. Namely, the absorbent material was notcompressed, and the compression ratio was 1. The results of measurementof the absorbent material are summarized in Table 3.

                  TABLE 3                                                         ______________________________________                                                 A     B       D      E     F    G                                    ______________________________________                                        Example 12 powders 6.55    42.5 11.3  25   --                                   Comp. powders 6.44 42.0 10.0 35 --                                            Example 4                                                                     Comp. Example 5 928 17.00 31.5 4.5 680 410                                    Example 13 powders 5.11 34.6 28.4 21 --                                       Comp. powders 5.22 34.0 27.8 32 --                                            Example 6                                                                     Comp. Example 7 963 19.22 22.4 9.8 580 380                                    Example 14 powders 5.11 45.3 27.4 25 --                                       Comp. Example 8 powders 0.5> 44.4 27.0 48 --                                ______________________________________                                         A: Weighing (g/m.sup.2)                                                       B: Water Content (% by Weight)                                                C: Absorbency (g/g)                                                           D: Absorbency under Load (g/g)                                                E: Absorbing Rate (seconds)                                                   F: Flexibility (mgf)                                                     

The results shown in Table 3 show that by reducing the water content ofthe water-containing gel under load, even when adopting the samewater-containing gel composition, compared with the case where the gelis heated without applying pressure, powdery absorbent material of stillimproved absorbency under load and absorbing rate can be obtained.Additionally, according to the conventional method, by forming it into asheet by once powdering them, the absorbing rate and absorbency underload are significantly reduced, and a sheet-like absorbent materialwhich shows excellent absorbing properties such as absorbency under loadand absorbing rate cannot be achieved.

Additionally, from the comparison between Example 14 and comparativeexample 8, it can be seen that as the resulting absorbent materialcontains compressed absorbent resin, by the distortion of thecross-linked structure of the absorbent resin, a still improvedabsorbing rate can be obtained.

EXAMPLE 15

In a reaction container equipped with a thermometer, a nitrogen gasinlet tube, an agitator, 1,000 g of 30 weight percent aqueous solutionof acrylic acid (monomer component) and sodium acrylate (monomercomponent) (75 mole percent of the total monomer components isneutralized) were added, and 1.63 g of the cross-linking agent (C) wereplaced to form a reaction solution, and nitrogen gas was introducedtherein to replace the reaction system with nitrogen gas.

While maintaining the reaction solution at 25° C.,2,2'-azobis(2-amidinopropane) diacrylate (hereinafter referred to as afoaming agent (E)) as the foaming agent was added thereto, and wasuniformly dispersed. Thereafter, in an atmosphere of nitrogen, sodiumpersulfate and L-ascorbic acid were added to carry out a polymerization.After an elapsed of time of around 10 minutes, foaming had started. Theamount of use of the foaming agent (E) with respect to 100 parts byweight of a solid portion of a monomer component is 0.2 percent byweight, and an amount of use of sodium persulfate with respect to themonomer component is 0.14 mole percent, and an amount of use ofL-ascorbic acid is 0.0008 mole percent.

After carrying out a polymerization for 60 minutes, the resultingbulk-like cellular gel (porous water-containing gel) that was expandedby around 1.05 times was cut into pieces so as to have a particlediameter of from 0.5 mm to 2 mm, thereby obtaining cellular gel having awater content of 70 percent by weight. The solid portion of the cellulargel was 30 percent by weight, and had an average porosity diameter of150 μm. Additionally, the BET specific area resulting from drying thecellular gel was 0.05 m² /g.

Then, 1,000 g of the cellular gel cut into fine pieces were placed intoa stainless lidded two-arm type kneader with two sigma blades and ajacket like that adopted in Example 1. On the other hand, a mixedsolution of 95 g of glycerol as polyhydric alcohol and 1.9 g of thesurface cross-linking agent (B) was prepared. Then, the cellular gel washeated by passing a hot water of 70° C. through the jacket withstirring. Thereafter, by adding the mixed solution to the cellular gelwith stirring, they were kept stirring to be uniformly mixed.

To the resulting mixture, 12.5 g of polyester fiber (subsidiary formingmaterial) made of polyethylene terephthalate having a length in a rangeof from 20 mm to 30 mm were gradually added, and were kneaded until clodno longer existed, thereby obtaining a cellular gel compositionincluding a cellular gel having a dried average particle diameter of 400μm.

Thereafter, the cellular gel composition was calendared using the samesingle drum dryer of Example 1, and the miller, and the surface incontact with the drum dryer of the cellular gel composition was heatedto 150° C. by the dryer drum. Then, the resulting sheet was scraped offby the scraper, thereby obtaining a flexible sheet-like absorbentmaterial that can be bent 180°.

The resulting absorbent material had a weight of 170 g/m², water contentof 9.0 percent by weight, absorbency of 20 g/g, absorbency under load of31 g/g and a absorbing rate of 13 seconds. The results of measurementsand essential conditions for manufacturing the cellular gel compositionare summarized in Table 4. By absorbing water, the cellular gel wasswollen to the original state before being milled, and the absorbentmaterial was curled in an opposite direction to the curved surface ofthe dryer drum.

EXAMPLE 16

The reaction and operations of Example 15 were repeated except that thefoaming agent (E) was not used when carrying out a polymerizationreaction, and non-cellular gel was obtained. Then, in the same manner asExample 15, a sheet-like absorbent material was obtained. The results ofmeasurements of the absorbent material and the essential conditions ofmanufacturing the water-containing gel composition of the presentinvention are summarized in Table 4.

EXAMPLE 17

An expansion polymerization was carried out in the same manner asExample 15 except that in place of the foaming agent (E), sodiumcarbonate (hereinafter referred to as a foaming agent (F)) was used inan amount of 2.5 percent by weight with respect to 100 parts by weightof a solid portion of the monomer component, and a surface active agentof polyoxyethylene sorbitan monostearate (hereinafter referred to asdispersion stabilizer (G)) was used as a dispersion stabilizer of thefoaming agent in an amount of 0.1 percent by weight with respect to 100parts by weight. Then, the resulting bulk-like cellular gel that wasexpanded to about two times (porous water-containing gel) was cut intofine pieces, thereby obtaining a cellular gel having a water content ofaround 70 percent by weight that was cut into pieces having a particlediameter in a range of from 0.5 mm and 2 mm. The solid portion of thecellular gel was 30 percent by weight, and had an average porositydiameter of 200 μm. The BET surface area of the dried cellular gel was0.05 m² /g.

Thereafter, 1,000 g of finely divided cellular gel were placed in akneader having the same arrangement as the kneader adopted in Example15. A mixed solution of 75 g of glycerol as a polyhydric alcohol and 1.5g of the surface cross-linking agent (B) were prepared. Next, thecellular gel was heated with stirring by passing hot water of 70° C.through the jacket. Thereafter, the resulting mixed solution was addedto the cellular gel until they were uniformly mixed.

To the resulting mixture, 13.5 g of polyester fibers (subsidiary formingmaterial) made of polyethylene terephthalate having a length in a rangeof from 20 mm to 30 mm were gradually added, and were kneaded until clodof the fibers no longer existed, thereby obtaining a cellular gelcomposition including a cellular gel having a dried average particlediameter of 350 μm.

Thereafter, the cellular gel composition was calendared and was scrapedoff by the scraper in the same manner as Example 15, thereby obtaining aflexible sheet-like absorbent material that can be bent 180°.

The resulting absorbent material had a weight of 300 g/m², water contentof 10.0 percent by weight, absorbency of 23 g/g, absorbency under loadof 27 g/g and a absorbing rate of 19 seconds. The results ofmeasurements and essential conditions for manufacturing the cellular gelcomposition were summarized in Table 4. By absorbing water, the cellulargel was swollen to the original state before being milled, and theabsorbent material was curled in an opposite direction to the curvedsurface of the drum dryer so as to have a low temperature surfaceinside.

EXAMPLE 18

The reaction and operations of Example 17 were repeated except that thefoaming agent (F) was not used when carrying out a polymerizationreaction, and non-cellular gel was obtained. Then, in the same manner asExample 17, a sheet-like absorbent material was obtained. The results ofmeasurements of the absorbent material and the essential conditions ofmanufacturing the water-containing gel composition of the presentinvention are summarized in Table 4.

                  TABLE 4                                                         ______________________________________                                                       EXAMPLE                                                                         15      16      17    18                                     ______________________________________                                        SOLID PORTION    30      30      30    30                                       (% BY WEIGHT)                                                                 CROSS-LINKING AGENT 1.63 1.63 1.63 1.63                                       (C) (g)                                                                       SURFACE CROSS-LINKING 1.9 1.9 1.5 1.5                                         AGENT (B) (g)                                                                 FOAMING AGENT (E) 0.2 --  -- --                                               (% BY WEIGHT)                                                                 FOAMING AGENT (F) -- -- 2.5 --                                                (% BY WEIGHT)                                                                 DISPERSION STABILIZER -- -- 0.1 0.1                                           (G) (% BY WEIGHT)                                                             GLYCEROL (g) 95 95 75 75                                                      POLYESTER FIBERS (g) 12.5 12.5 13.5 13.5                                      (mm) 20-30 20-30 20-30 20-30                                                  WEIGHT (g/m) 170 175 300 310                                                  WATER CONTENT 9.0 9.5 10.0 11.1                                               (% BY WEIGHT)                                                                 ABSORBENCY (g/g) 20 20 23 23                                                  ABSORBENCY UNDER 31 30 27 26                                                  LOAD (g/g)                                                                    ABSORBING RATE (seconds) 13 40 19 45                                          FLEXIBILITY (mgf) 480 490 560 570                                           ______________________________________                                    

As is clear from the results shown in Table 4, the absorbent materialresulting from the manufacturing method of the present invention hadexcellent properties especially in absorbency under load and absorbingrate. The results also proved that the absorbent materials produced fromthe cellular gel as a gel were superior to those produced from thenon-cellular gel in their absorbing rate, absorbency under load, etc.

EXAMPLE 19

The reaction and the operations of Example 4 were repeated, and asheet-like absorbent material was obtained. Then, the resultingabsorbent material was cut into a piece of 12 cm×25 cm size. On theother hand, non-woven fabrics were taken out from paper diapers (PampersL-size (product name) available from Proctor & Gamble (P&G) Co.), andthe non-woven fabrics were cut into the same size as the absorbentmaterial. Then, the non-woven fabrics were laminated on the absorbentmaterial, thereby obtaining a simple absorbing member as an example ofthe absorbent article.

The performances of the resulting simple absorbing member weredetermined. Specifically, the simple absorbing member was placed on anacryl plate, and another acryl plate was placed on the simple absorbentmember. The upper acryl plate had a liquid introducing tube having aninner diameter of 23 mm at a position corresponding to a central portionof the simple absorbent member. Then, with respect to the simpleabsorbent member, a load of 23 g/cm² was uniformly applied to the simpleabsorbent member.

In this state, 50 ml of physiologic saline solution was poured into aliquid tube, and the time required from the state till the completion ofabsorbing the physiologic saline by the absorbent in a simple manner wasmeasured, to set the time as the absorbing time (seconds). Then, theoperation was repeated three times every 5 minutes, and after an elapseof 5 minutes from the completion of the third operation, the upper acrylplate and the weight were taken out, and 10 pieces of kitchen towels(Nepia, available from Shin Oji Paper Co., Ltd.) were placed on thesample absorbing member. Thereafter, the upper acryl plate and theweight were placed thereon again. The weight of the kitchen towel wasmeasured beforehand.

After an elapse of 1 minutes from the completion of the describedoperation, the weight of the kitchen towel was measured. The weightobtained by extracting the original weight from the weight after theoperation was determined to be a return weight (g).

The absorbing times of the first, second and third operations were 535seconds, 1185 seconds and 610 seconds respectively, and the returnweight was 11.97 g.

Possible Industial Use of the Invention

The absorbent resin and the absorbent material of the present inventionhave excellent absorbing properties such as absorbing rate, absorbencyunder load and shape retaining property. For these beneficial features,the absorbent resin and the absorbent material of the present inventionare suited for use in absorbent articles, for example, sanitarymaterials such as paper diapers (disposable diapers), sanitary napkins,so-called incontinence pads, etc., moisture condensation absorbentsheets, which are desired to have higher performances and to be madethinner. Therefore, the present invention offers the described absorbentarticles of excellent performances.

We claim:
 1. An absorbent resin which is characterized by anisotropically swelling against distortion from a compressed and distorted state into a non-similar shape by absorbing an aqueous solution and which is made of a water-containing gel capable of absorbing an aqueous fluid in an amount of not less than three times the weight of the gel.
 2. The absorbent resin as set forth in claim 1, characterized by being compressed at a ratio of compression in a range of from 2 to 1,000.
 3. The absorbent resin as set forth in claim 2, characterized by having a distorted cross-linked structure by compression.
 4. An absorbent resin as defined in claim 2, characterized by restoring its original shape before being compressed.
 5. The absorbent resin as set forth in claim 1, characterized by having foams inside.
 6. An absorbent material comprising:an absorbent resin which anisotropically swells against distortion from a compressed and distorted state into a non-similar shape by absorbing an aqueous solution and which is made of a water-containing gel capable of absorbing an aqueous fluid in an amount of not less than three times the weight of the gel, and being formed into a sheet so as to have a flexibility of not more than 1,000 mgf.
 7. The absorbent material as set forth in claim 6, characterized by being swollen by absorbing water so as to have a curvature.
 8. A method of manufacturing an absorbent material, comprisingreducing an amount of an aqueous solvent in a water-containing gel of a hydrophilic cross-linked polymer while compressing said water-containing gel to produce an absorbent resin which anisotropically swells against distortion from a compressed and distorted state into a non-similar shape by absorbing an aqueous solution, said water-containing gel being capable of absorbing an aqueous fluid in an amount of not less than three times the weight of the gel and, molding said adsorbent resin into a sheet.
 9. The method of manufacturing an absorbent material as set forth in claim 8, characterized in that:said water-containing gel further includes polyhydric alcohol.
 10. The method of manufacturing an absorbent material as set forth in claim 8, characterized in that a water content of said water-containing gel is in a range of from 30 percent by weight to 90 percent by weight.
 11. The method of manufacturing as set forth in claim 8, characterized by including the step of calendaring said water-containing gel to be formed into a sheet.
 12. The method of manufacturing an absorbent material a set forth in claim 11, characterized by carrying heating and pressurizing processes simultaneously so as to generate a temperature difference between a first surface and a second surface of the sheet.
 13. A method of manufacturing an absorbent material, comprisingcompressing a cellular gel of a hydrophilic cross-linked polymer to produce an absorbent resin which anisotropically swells against distortion from a compressed and distorted state into a non-similar shape by absorbing an aqueous solution, said cellular gel being a water-containing gel capable of absorbing an aqueous fluid in an amount of not less than three times the weight of the gel and, molding said absorbent resin into a sheet.
 14. The method of manufacturing the absorbent material as set forth in claim 13, characterized in that:said cellular gel further comprises polyhydric alcohol.
 15. The method of manufacturing the absorbent material as set forth in claim 13, characterized in that:a water content of said cellular gel is in a range of from 30 percent by weight to 90 percent by weight.
 16. The method of manufacturing an absorbent material as set forth in claim 13, characterized in that:an amount of aqueous solvent in said cellular gel is reduced under an applied pressure.
 17. The method of manufacturing an absorbent material as set forth in claim 13, characterized by comprising the step of calendaring said cellular gel to be formed into a sheet.
 18. The method of manufacturing an absorbent material as set forth in claim 17, characterized by comprising the step of carrying out pressurizing and heating processes simultaneously so as to generate a temperature difference between a first surface and a second surface of the sheet.
 19. The absorbent resin as set forth in claim 1, characterized by being prepared by compressing a water-containing gel of a hydrophilic cross-linked polymer.
 20. The absorbent resin as set forth in claim 19, characterized in that:the water-containing gel is a gel-like hydrophilic cross-linked polymer prepared by polymerizing a monomer component including an ethylenically unsaturated monomer using an aqueous solvent.
 21. The absorbent resin as defined in claim 20, characterized in that:said hydrophilic cross-linked polymer has a three-dimensional net structure, an inside portion thereof being cross-linked by a cross-linking agent, and said cross-linking agent is used in an amount ranging from 0.001 mole percent to 2 mole percent based on the amount of the monomer component.
 22. The absorbent resin as set forth in claim 19, characterized in that:said hydrophilic cross-linked polymer is a cross-linked poly(meth)acrylic acid (salt).
 23. The absorbent resin as defined in claim 19, characterized in that the water-containing gel includes a water-soluble component in a range of from 0.1 percent by weight to 20 percent by weight.
 24. The absorbent resin as defined in claim 2, characterized in that:said compression ratio is Y/X, wherein X is a compression ratio in a direction of compressing the absorbent resin, and Y is an expansion ratio in an expanding direction of the absorbent resin.
 25. An absorbent material as defined in claim 6, characterized in that said absorbent resin is prepared by compressing a water-containing gel of a hydrophilic cross-linked polymer.
 26. The absorbent material as set forth in claim 25, characterized in that:the water-containing gel is a gel-like hydrophilic cross-linked polymer prepared by polymerizing a monomer component including an ethylenically unsaturated monomer using an aqueous solvent.
 27. The absorbent material as defined in claim 26, characterized in that:said hydrophilic cross-linked polymer has a three-dimensional net structure, an inside portion thereof being cross-linked by a cross-linking agent, and said cross-linking agent is used in an amount ranging from 0.001 mole percent to 2 mole percent based on the amount of the monomer component.
 28. The absorbent material as set forth in claim 25, characterized in that said hydrophilic cross-linked polymer is a cross-linked poly(meth)acrylic acid (salt).
 29. The absorbent material as defined in claim 25, characterized in that:said water-containing gel has cells inside.
 30. The absorbent material as defined in claim 25, characterized in that:said water-containing gel includes a water-soluble component in a range of from 0.1 percent by weight to 20 percent by weight.
 31. The absorbent material as defined in claim 25, characterized in that:a compression ratio of said water-absorbent resin is in a range of from 2 to 1,000.
 32. The absorbent material as defined in claim 31 characterized in that:said compression ratio is Y/X, wherein X is a compression ratio in a direction of compressing the absorbent resin, and Y is an expansion ratio in an expanding direction of the absorbent resin.
 33. The absorbent material as set forth in claim 6, characterized in that:an absorbing rate of the material is not less than 150 seconds.
 34. The absorbent material as set forth in claim 6, characterized in that:said material has an absorbency under pressure is not less than 15 g/g.
 35. The method of manufacturing an absorbent material as set forth in claim 8, characterized in that:from 10 to 90 percent by weight of an aqueous solvent of the water-containing gel of the hydrophilic cross-linked polymer is reduced before being compressed.
 36. The method of manufacturing an absorbent resin as defined in claim 8, characterized in that:a conversion of said water-containing gel is in a range of from 90 to 99.99%.
 37. The method of manufacturing an absorbent material as set forth in claim 8, characterized in that the water-containing gel is a gel-like hydrophilic cross-linked polymer prepared by polymerizing a monomer component including an ethylenically unsaturated monomer using an aqueous solvent.
 38. The method of manufacturing an absorbent material as set forth in claim 37, characterized in that:said hydrophilic cross-linked polymer has a three-dimensional net structure, an inside portion thereof being cross-linked by a cross-linking agent, and said cross-linking agent is used in an amount ranging from 0.001 mole percent to 2 mole percent based on the amount of the monomer component.
 39. The method of manufacturing an absorbent material as set forth in claim 8 characterized in that:said hydrophilic cross-linked polymer is a cross-linked poly(meth)acrylic acid (salt).
 40. The method of manufacturing an absorbent material as set forth in claim 8, characterized in that:the water-containing gel includes a water-soluble component in a range of from 0.1 percent by weight to 20 percent by weight.
 41. The method of manufacturing an absorbent material as set forth in claim 13, characterized in that:a conversion of said cellular gel is in a range of from 90 percent to 99.99 percent.
 42. The method of manufacturing an absorbent material as set forth in claim 13, characterized in that said cellular gel is a gel-like hydrophilic cross-linked polymer prepared by polymerizing a monomer component including an ethylenically unsaturated monomer using an aqueous solvent in a presence of a foaming agent.
 43. The method of manufacturing an absorbent material as set forth in claim 42, characterized in that:said hydrophilic cross-linked polymer has a three-dimensional net structure, an inside portion thereof being cross-linked by a cross-linking agent, and said cross-linking agent is used in an amount ranging from 0.001 mole percent to 2 mole percent based on the amount of the monomer component.
 44. The method of manufacturing an absorbent material as set forth in claim 13, characterized in that:said hydrophilic cross-linked polymer is a cross-linked poly(meth)acrylic acid (salt).
 45. The method of manufacturing an absorbent material as set forth in claim 13, characterized in that:the water-containing gel includes a water-soluble component in a range of from 0.1 percent by weight to 20 percent by weight.
 46. The method of manufacturing an absorbent material as set forth in claim 16, characterized in that:from 10 to 90 percent by weight of an aqueous solvent of a water-containing gel of the hydrophilic cross-linked polymer before being compressed is reduced. 